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

[Brad King via cfe-commits]

bradking wrote:

@DavidTruby thanks.  This LGTM now.  I've locally updated CMake to use the preprocessor definitions in place of the workaround we had before, and it seems to work with this  PR.


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