[flang][cmake] Improve how CLANG_DIR is handled

* Added a sanity check with `Clang_FOUND` to verify that find_package
succeeded
* Made sure that find_package won't use any of CMake's standard paths
to guarantee that only the path provided with CLANG_DIR is considered
(implemented through NO_DEFAULT_PATH)
* Made the call to get_filename_component more explicit (so that it is
clear what the base directory is)
* Updated comments to clarify what CLANG_DIR means

Differential Revision: https://reviews.llvm.org/D99088