Add an LLVM_PTR_SIZE macro to make LLVM_ALIGNAS more useful

Add an LLVM_PTR_SIZE macro to make LLVM_ALIGNAS more useful

MSVC 2013 requires the argument to __declspec(align()) to be an integer
constant expression that doesn't involve any identifiers like sizeof.

For GCC and Clang, LLVM_PTR_SIZE is equivalent to __SIZEOF_POINTER__,
which dates back to GCC 4.6 and Clang 2010. If that's not available, we
get sizeof(void*), which works with alignas() and
__attribute__((aligned())).

For MSVC, LLVM_PTR_SIZE is 4 or 8 depending on _WIN64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233929 91177308-0d34-0410-b5e6-96231b3b80d8