LLVMFlang: Update MSVC ABI support for LLVM 18.0 development version

As of llvm-project main branch commit 86accd4e03 (2023-12-04), LLVMFlang 18.0.0 has integrated additional support for the MSVC ABI:

• LLVM PR 72519: MSVC runtime library selection
• LLVM PR 73250: MSVC predefines

These resolve some of the FIXME comments left by !8852 (merged).

Issue: #24840 (closed)