[libcxx] Check MSVC flag directly instead of setting LIBCXX_TARGETING_MSVC

MSVC is set for actual msvc and clang-cl. LIBXX_TARGETTING_CLANG_CL is
still used over in runtimes to configure warning flags.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D154698