Make LLVM/clang detection compatible with vcpkg

vcpkg is a C++ package manager for Windows, Linux, and MacOS. It is especially convenient on Windows, where there is no standard package manager to install things like OpenSSL, mysql, and you often to provide different compilations per toolchain / compiler version.

We are currently looking into making vcpkg a standard way to provide third-party dependencies in Qt.

vcpkg can also install llvm 6.0 by just running e.g.

 vckpkg install llvm:x64-windows

This will generate following structure:

%VCPKG_DIR%\installed\x64-windows\include\clang\*
%VCPKG_DIR%\installed\x64-windows\include\clang-c\*
%VCPKG_DIR%\installed\x64-windows\include\llvm\*
%VCPKG_DIR%\installed\x64-windows\include\llvm-c\*%VCPKG_DIR%\installed\x64-windows\lib\*
%VCPKG_DIR%\installed\x64-windows\lib\clang\*
%VCPKG_DIR%\installed\x64-windows\tools\llvm\*

 
With llvm-config.exe in tools\llvm:

%VCPKG_DIR%\installed\x64-windows\tools\llvm\llvm-config.exe

It seems like the compiled llvm version should be fit to be used by qdoc. Anyhow, this does not match the current logic in qt_find_clang.prf is hardcoded for a different setup, where everything needs to be under a common LLVM_INSTALL_DIR, and also llvm-config is looked up in %LLVM_INSTALL_DIR%\bin.