This eases consumption of the LibraryImport source generator by not forcing the use of DisableRuntimeMarshalling for types where we can determine the type is blittable based on the following rules:

1. The type is "always blittable" (i.e., int or double, but not char).
2. A value type defined in the source project the current source generator is running on.
3. Is a type made up of types adhering to (1) and (2).

This will permit marshalling of the following:

public struct S
{
    public short A;
    public short B;
}

[CustomTypeMarshaller(typeof(S))]
public struct Native
{
    private short a;
    private short b;
    public Native(S s) { ... }
    public S ToManaged() { ... }
}

Fixes #69033

/cc @dotnet/interop-contrib