[libclc] [libclc][CMake] Allow empty CMAKE_C_COMPILER_ID when CMAKE_C_COMPILER_WORKS is set (PR #186717)

[Wenju He via cfe-commits]

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@@ -24,7 +24,21 @@ if(NOT CMAKE_CLC_COMPILER)
     message(FATAL_ERROR "The CLC language requires Clang. ${reason}.")
   endif()
 
-  set(CMAKE_CLC_COMPILER "${CMAKE_C_COMPILER}" CACHE FILEPATH "CLC compiler")
+  # Use the regular clang driver if the C compiler is clang-cl.
+  if(CMAKE_C_COMPILER_ID STREQUAL "Clang" AND CMAKE_C_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC")
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wenju-he wrote:

> I'm not sure what windows or clang-cl has to do with anything. There aren't any host side components built anymore here?

windows or clang-cl check is reverted in https://github.com/llvm/llvm-project/pull/191745. Yes, there is no host side component anymore

https://github.com/llvm/llvm-project/pull/186717