[flang] [clang] [flang] Add MSC_VER and target arch defines when targeting the MSVC ABI (PR #73250)

[David Truby via cfe-commits]

DavidTruby wrote:

@pbo-linaro @bradking I think this fixes the linked issue, could someone confirm that for me? Thanks!

https://github.com/llvm/llvm-project/pull/73250