Clang: Default to llvm-rc for CMAKE_RC_COMPILER
This change sets the default resource compiler to llvm-rc when building with Clang on the Windows platform. This was implemented by copying similar logic from CYGWIN-GNU.cmake, Windows-GNU.cmake and Windows-MSVC.cmake into Windows-Clang.cmake. With this change in place, trivial C/C++ programs can now be generated with Ninja and Clang on Windows without running into any resource compiler errors.
Fixes: #19318 (closed)