LLVMFlang: Update MSVC runtime library selection for LLVMFlang 18.0 LLVMFlang 18.0 adds MSVC runtime library selection flags and associated Fortran runtime library variants. 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