[clang] Add AddClang.cmake to the list of the CMake modules that are installed

This makes sure that AddClang.cmake is installed alongside other Clang
CMake modules. This mirrors LLVM and MLIR in this respect and is
required when building the new Flang driver out of tree (as it depends
on Clang and includes AddClang.cmake).

Reviewed By: bogner

Differential Revision: https://reviews.llvm.org/D94533