[libclc] [llvm] [libclc][CMake] Use clang/llvm-ar on Windows (PR #186726)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> This is not working because CMake would use MSVC to make the static library.

I thought we had code in `libclc` to configure the correct `ar`. Could look into that.

How difficult would it be to just configure this project as an extra build like how the runtimes builds do it? That would probably look something like this:
```cmake
  include(ExternalProject)
  ExternalProject_Add(libclc-build
    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/llvm/runtimes
    CMAKE_ARGS
      -DLLVM_ENABLE_RUNTIMES=libclc
      -DCMAKE_C_COMPILER=${CMAKE_BINARY_DIR}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}
      -DCMAKE_CXX_COMPILER=${CMAKE_BINARY_DIR}/bin/clang++${CMAKE_EXECUTABLE_SUFFIX}
      -DCMAKE_C_COMPILER_WORKS=ON
      -DCMAKE_CXX_COMPILER_WORKS=ON
      -DLLVM_BINARY_DIR=${CLSPV_LLVM_BINARY_DIR}
      -DLIBCLC_TARGETS_TO_BUILD=clspv--$<SEMICOLON>clspv64--
    DEPENDS clang llvm-link opt llvm-ar llvm-ranlib
    BUILD_ALWAYS ON
    INSTALL_COMMAND ""
  )
```

https://github.com/llvm/llvm-project/pull/186726