Add tests for ANSI BSTRs (#20985)

Add tests for ANSI BSTRs (#20985)

* Add tests for ANSI BSTRs

* Fix signed/unsigned mismatch in comparisons. Add xplatform include

* Use regular memcpy

* Enable ANSI BSTR off-windows.

* Also enable UnmanagedType.TBStr off-Windows since both BStr variants are available off-Windows.

* Make sure the AnsiBSTR marshaller is hooked up off-Windows.

* Remove a few more ifdefs I missed.

* remove ifdefs around binder definitions and the C# stub helpers.

* PR Feedback.

* Add xplat ARRAYSIZE macro

* Move xplat ARRAYSIZE to platformdefines.h