clang_installation.pri
* Remove pointless CLANG_COMPLETION, CLANG_HIGHLIGHTING defines
* Remove auto-detection of llvm-config-* variants in PATH. This does
  not make any sense if the user has to provide LLVM_INSTALL_DIR.
* If llvm-config is not found in LLVM_INSTALL_DIR, determine the clang
  version by looking into the lib dir (there should exist e.g.
  "clang/3.6.1")
* Check paths for existence
* Simplify logic
* Do not call error() in helper functions

clangcodemodel.pro
* Print detected version
* Sort HEADERS/SOURCES

Change-Id: I020d2e78aea37ef01a337e6da658d7f5e56b3ac5
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>