The find_first_existing_file and find_first_existing_vc_file macros in lib/Basic/CMakeLists.txt are removed. The macros are also defined in {LLVM}/cmake/modules/AddLLVM.cmake for the same purpose. This change serves the following 2 objectives:
- To remove the redundant code in clang to use the same macros in llvm,
- The macros in AddLLVM.cmake can also handle repo for displaying correct version information.