CMake: allow generation of a Qt Compressed Help file for Clang source

r203272 enables generation of a Qt Compressed Help file for the LLVM source
code.

As a consequence, this change brings the same capabilities to Clang while
reusing these two variables:

  LLVM_ENABLE_DOXYGEN_QT_HELP
  LLVM_DOXYGEN_QHELPGENERATOR_PATH

and adding

  CLANG_DOXYGEN_QCH_FILENAME="org.llvm.clang.qch"
  CLANG_DOXYGEN_QHP_NAMESPACE="org.llvm.clang"
  CLANG_DOXYGEN_QHP_CUST_FILTER_NAME="Clang ${CLANG_VERSION}"
  CLANG_DOXYGEN_QHP_CUST_FILTER_ATTRS="Clang,${CLANG_VERSION}"

Patch by Konrad Kleine.