Enable returning more complex structures via PInvoke returns. (#21470)

Enable returning more complex structures via PInvoke returns. (#21470)

* Add test verifying behavior in dotnet/coreclr#19676.

* Clean up test code.

* Test what happens if we enable returning structures by value.

* Use braced initializer.

* Update Decimal tests to expect that returning a decimal by LPStruct or Currency works.

* Change handle-in-struct marshalling to expect a NotSupportedException thrown at marshal time instead of expecting a MarshalDirectiveException at signature time.

* Update Decimal Reverse-PInvoke tests.

* Disable some previously disabled return marshalling types and add a nice comment block explaining why they're disabled.

* Enable marshalling DateTime return values and add a test.

* Rename IsUnuspportedValueTypeReturn

* Add return test for ArrayWithOffset

* Remove extraneous P/Invoke.

* Fix spelling.

* Add test for successfully returning a struct that has one field of a type that is return-type blocked.

* Add explicit struct return test.

* Clean up tests.

* Fix grammer.

* Add test for struct whose managed layout doesn't require a stdcall return buffer but whose native layout does.

* Add test verifying HandleRef behavior.

* Clean up IsUnsupportedTypedefReturn per PR feedback.