[libc++] Default to vcruntime when targeting MSVC

Summary:
libc++abi is never the right option for LIBCXX_TARGETING_MSVC, since it
targets the Itanium ABI, whereas MSVC uses the Microsoft ABI. Make the
default ABI be vcruntime when targeting MSVC even if libc++abi is
present in the tree.

Differential Revision: https://reviews.llvm.org/D32320

llvm-svn: 300921