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)