Fixes dotnet#8595 by storing the embedded-file zip in a temporary directory
(instead of memory or binlog target directory) to avoid problems with
file watchers.

In a recent profile of a graph construction, it was observed that a
large amount of boxing was happening for
ProjectElementSiblingEnumerable. This change simplifies how xml children
are enumerated by adding an internal ChildrenEnumerable property which
directly exposes the ProjectElementSiblingEnumerable which should avoid
boxing, at least in some code paths (the public API makes it hard to
avoid everywhere...).

Additionally, a very common usage of enumerating children was to do
Children.OfType<T> and wrap it in a ReadOnlyCollection<T>, so I
introduced a GetChildrenOfType (and GetChildrenReversedOfType) method
which exposes an ICollection<T> which does the same thing but without
the boxing of ProjectElementSiblingEnumerable and without the OfType
class. It's just 1 collection allocation.

Currently we turn off dynamic platform resolution for a whole solution
if a single project in the solution is assigned a configuration. This is
problematic as some projects are outside of the scope of the solution
but still have certain targets that run on them that are architecture
specific. These projects will build as the wrong architecture because no
configuration is defined and no platform negotiation takes place.

I removed the conditional that turns platform negotiation off on a sln
level. The logic to turn this off on a project level is already in place
through checking is a projectreference has setplatform appended to it.
This will make sure no projects with configurations defined will be
negotiated for as MSbuild adds setplatform metadata to projectreferences
with configurations.

…-enumeration', 'backport/pr-8870-to-vs17.7' and 'dev/michaelshea/sln' into vs17.7