[cmake] Adjust for non-installed LLVM
In LLVM 16 llvm-config got --src-root removed. Adjust include path for Clang include
Tested locally with LLVM 18.1.0rc.
In LLVM 16 llvm-config got --src-root removed. Adjust include path for Clang include
Tested locally with LLVM 18.1.0rc.