* Move Clang*.cmake back to /usr/lib/llvm-X.Y/lib/cmake/clang and
install a symlink in /usr/lib/cmake/clang-X.Y to ensure that CMake can
still find it.
* Ensure that the LLVM installation prefix is correctly discovered
despire symlinks (replaces fix-cmake-config-prefix.diff).
* Create /usr/lib/llvm-X.Y/bin/clang-6.0 symlink as required by
ClangTargets-relwithdebinfo.cmake.
* Remove useless LLVM_CMAKE_DIR sed command that did not match anything.
* Ignore missing binaries in ClangTargets-relwithdebinfo.cmake.
* Add regression test to qualify-clang.sh.
