C++: Enable runtime lookup debug...

... by setting the environment variable QTC_LOOKUPCONTEXT_DEBUG

Change-Id: Ia46f55b2989129d5327d894734b6ea9c77c3e0ac
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
ClangCodeModel: fixed build on windows for newer LLVM installs

Latest LLVM release has Windows installer which installs it to Program Files.

Change-Id: I55d7e43d1333f3a7280020e0ebcb1e095acca9d0
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>