look for clang-cpp shared lib when it exists

[andrewrk]

This is a much less invasive change than #4992. It makes zig build successfully against homebrew's clang package.

#4992 is blocked on an apt.llvm.org packaging issue, as well as the docker images giving this error:

Configuring zig version 0.5.0+2be2bcdcb
CMake Error at /deps/local/lib/cmake/llvm/LLVMExports.cmake:1293 (message):
  The imported target "llvm-tblgen" references the file

     "/deps/local/bin/llvm-tblgen"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "/deps/local/lib/cmake/llvm/LLVMExports.cmake"

  but not all the files it references.

Call Stack (most recent call first):
  /deps/local/lib/cmake/llvm/LLVMConfig.cmake:256 (include)
  /deps/local/lib/cmake/clang/ClangConfig.cmake:10 (find_package)
  CMakeLists.txt:66 (find_package)

So the cmake-based configuration is really a no-go. I also don't like how non-standard that is.

See #4799. The homebrew parts of this are solved by this PR, however, the issue remains open due to https://bugs.llvm.org/show_bug.cgi?id=44870.