LLVMFlang: Add support for targeting MSVC ABI on Windows

The compiler does not yet support everything needed to integrate well
with the MSVC ABI, in particular for runtime library selection and debug
format selection.  Document them in FIXME comments and leave this
support undocumented by CMake for now.

Fixes: #24840
Inspired-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
8 files changed