[clang] [compiler-rt] [llvm] [CMake] Use Clang to infer the target triple (PR #89425)

[via cfe-commits]

BukeBeyond wrote:

The following code breaks builds on Windows:

compiler-rt/cmake/Modules/CompilerRTUtils.cmake:387
runtimes/CMakeLists.txt:197
```
 if (CMAKE_C_SIMULATE_ID MATCHES "MSVC")
    set(option_prefix "/clang:")
 endif()
```

The problem is when LLVM is built with Clang-cl, CMAKE_C_SIMULATE_ID gets MSVC.  But these sub builds switch the compiler to the newly built clang++ without updating CMAKE_C_SIMULATE_ID, and this condition sets an invalid flag "/clang:".

https://github.com/llvm/llvm-project/pull/89425