[libcxx] [test] Add a separate 'windows-dll' feature to check for

This allows distinguishing failures in tests that only fail when libcxx
is linked as a DLL, allowing narrowing down XFAILs (avoiding XPASS errors
if not built as a DLL).

If both enable_shared and enable_static are set, the tests link and use
the shared version of the lib.

Differential Revision: https://reviews.llvm.org/D100221