Add PInvoke/Structures tests

Add PInvoke/Structures tests

Fix compile warnings

Fix compile warnings

Fix compile warnings

Remove duplicate tests. Move ReversePinvoke tests under StructMarshalling.

Clean up some compile errors.

More string cleanup.

More string fixes

Clean up consts and typedefs.

Fix typo.

Change TP_slen return type to match xplatform.h

Fix printf specifiers.

Don't define wcslen in xplatform if it's been macro redefined (to platformdefines.h's TP_slen)

Fix build breaks.

Remove extension from DllImport so it will correctly resolve xplat.

Fix printf specifiers again...

Fix Windows x86 calling convention mismatch.

Fix HRESULT size off-Windows

Commit migrated from https://github.com/dotnet/coreclr/commit/aa0d4daf1491562a6c7e6ef18f719938a0e6e4bd