[libc++] Allow users to explicitly specify ABI

libc++'s current heuristic for detecting Itanium vs. Microsoft ABI falls
short in some cases. For example, it will detect windows-itanium targets
as using the Microsoft ABI, since they set `_MSC_VER` (for compatibility
with Microsoft headers). Leave the current heuristic in place by default
but also allow users to explicitly specify the ABI if need be.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@314949 91177308-0d34-0410-b5e6-96231b3b80d8