This corrects the size on disk regression caused by dotnet#19912. After that pull request, all of the schema validation stuff would always be included for any app that depends on System.Private.Xml because during initial analysis we consider a lot of things in S.P.Xml necessary as a precaution for SG generating references to it. The RD.XML in S.P.Xml then roots things forever.

I spent some time getting a repro of the original failure and convinced myself this is not a scenario that would require RD.XML in the S.P.Xml assembly.

Here's the facts:
* This test uses a test hook to explicitly disable pregenerated serialization code for `XmlSchema`. (https://github.com/dotnet/corefx/blob/1afc5360013bedc4099875c836342f493b083e5f/src/System.Private.Xml/src/System/Xml/Schema/XmlSchema.cs#L175 that is on stack at the time of failure is an analysis slam dunk, so we do have pregenerated code, we just don't use it because of the hook)
* The test is then testing the reflection fallback path to serialize `XmlSchema`. This doesn't work. Reflection fallback path won't in general work for _any framework provided type_ because there's no RD.XML to root it. Reflection fallback really only works for user types because of our default RD.XML that roots all user types (we also use that for the CoreFX tests, which is why the other reflection fallback tests don't hit issues).

* Fix RD.XML for System.Private.Xml (#31125)

This corrects the size on disk regression caused by #19912. After that pull request, all of the schema validation stuff would always be included for any app that depends on System.Private.Xml because during initial analysis we consider a lot of things in S.P.Xml necessary as a precaution for SG generating references to it. The RD.XML in S.P.Xml then roots things forever.

I spent some time getting a repro of the original failure and convinced myself this is not a scenario that would require RD.XML in the S.P.Xml assembly.

Here's the facts:
* This test uses a test hook to explicitly disable pregenerated serialization code for `XmlSchema`. (https://github.com/dotnet/corefx/blob/1afc5360013bedc4099875c836342f493b083e5f/src/System.Private.Xml/src/System/Xml/Schema/XmlSchema.cs#L175 that is on stack at the time of failure is an analysis slam dunk, so we do have pregenerated code, we just don't use it because of the hook)
* The test is then testing the reflection fallback path to serialize `XmlSchema`. This doesn't work. Reflection fallback path won't in general work for _any framework provided type_ because there's no RD.XML to root it. Reflection fallback really only works for user types because of our default RD.XML that roots all user types (we also use that for the CoreFX tests, which is why the other reflection fallback tests don't hit issues).

* Add RemovableFeatureAttribute

* Make non-file URL support optional in XmlUrlResolver

This adds [removable feature annotation](dotnet/designs#42) to `XmlDownloadManager`.

If the user specifies that they would like to remove support for this at publish/native compilation time, the linker/compiler is going to replace the method body of `CreateWebRequestOrThrowIfRemoved` with a throwing method body. The exception message is going to inform the user that the feature has been removed because they opted into the removal.

Contributes to #30597. Saves 1.2 MB of the size of the Windows UWP People app. This is a size on disk regression that came with NetStandard 2.0.

* Annotate reflection fallback for S.P.DataContractSerialization (#31533)

This adds [removable feature annotation](dotnet/designs#42) to data
contract serialization.

If the user specifies that they would like to remove support for this
at publish/native compilation time, the linker/compiler is going to
replace the annotated method bodies with a throwing method body.
The exception message is going to inform the user that the feature
has been removed because they opted into the removal. The throwing
method body is significantly smaller than the transitive closure
of code reachable from the original method body.

Contributes to #30597. Turning this feature off in the UWP People app
saves 4% of size. This is a size on disk regression that came with
the new version of CoreFX and blocks the Windows team in picking up
the latest framework. There is a zero size growth goal.

* Sync SR class with the CoreRT copy

This ports over the change from
dotnet/corert@0ac83cb.

When UsingResourceKeys is true and we stripped the resources, the
existing code would have returned null.

This corrects the size on disk regression caused by dotnet/corefx#19912. After that pull request, all of the schema validation stuff would always be included for any app that depends on System.Private.Xml because during initial analysis we consider a lot of things in S.P.Xml necessary as a precaution for SG generating references to it. The RD.XML in S.P.Xml then roots things forever.

I spent some time getting a repro of the original failure and convinced myself this is not a scenario that would require RD.XML in the S.P.Xml assembly.

Here's the facts:
* This test uses a test hook to explicitly disable pregenerated serialization code for `XmlSchema`. (https://github.com/dotnet/corefx/blob/dotnet/corefx@1afc5360013bedc4099875c836342f493b083e5f/src/System.Private.Xml/src/System/Xml/Schema/XmlSchema.cs#L175 that is on stack at the time of failure is an analysis slam dunk, so we do have pregenerated code, we just don't use it because of the hook)
* The test is then testing the reflection fallback path to serialize `XmlSchema`. This doesn't work. Reflection fallback path won't in general work for _any framework provided type_ because there's no RD.XML to root it. Reflection fallback really only works for user types because of our default RD.XML that roots all user types (we also use that for the CoreFX tests, which is why the other reflection fallback tests don't hit issues).

Commit migrated from dotnet/corefx@6867b6f