LLVMFlang: Update MSVC ABI and architecture detection for LLVMFlang 18.0

LLVMFlang 18.0 adds MSVC ABI and architecture macros.  Resolve the
corresponding FIXME left by commit 26bf32cdc6 (LLVMFlang: Add support
for targeting MSVC ABI on Windows, 2023-09-28, v3.28.0-rc1~10^2).

Issue: #24840
2 files changed