Add _LIBCPP_ABI_[ITANIUM|MICROSOFT] macros.

This patch adds a libc++ configuration macro for the ABI we
are targeting, either Itanium or Microsoft. For now we configure
for the Microsoft ABI when on Windows with a compiler that defines
_MSC_VER. However this is only temporary until Clang implements
builtin macros we can use.

llvm-svn: 291329