[clad] Specify where the clang header files are.

In cases where we build ROOT with -Dbuiltin_llvm=Off -Dbuiltin_clang=On
and we have installed both llvm and clang in /usr/ clad will pick up
the clang headers from there too.

This patch gives higher priority to the header files which ROOT is
supposed to use. It fixes a very obscure initialization issue due to
different versions of the ASTContext.h installed and used by ROOT.