Make Clang cmake files more usable

• 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.

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Hi Sylvestre,

This should fix https://bugs.llvm.org/show_bug.cgi?id=37128 (WIP due to the issue below). Let me know what you think of this approach. I can push the two CMakeLists.txt patches upstream once reviewed, then they can be dropped for 7.x.x (and maybe 6.x as well if it can be backported).

TODO:

• dpkg-buildpackage test - successful
• debian/changelog?
• workaround fatal error when not all clang binaries are not fully installed (see below)
• update qualify-clang.sh script

Make Error at /usr/lib/llvm-6.0/lib/cmake/clang/ClangTargets.cmake:490 (message):
  The imported target "clang-format" references the file

     "/usr/lib/llvm-6.0/bin/clang-format"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "/usr/lib/llvm-6.0/lib/cmake/clang/ClangTargets.cmake"

  but not all the files it references.