Create a FindClang.cmake and move all Clang configuration there.
Find Clang and choose static libraries if available, otherwise use
clang-cpp if available.
Add option to link to clang-cpp even if static libraries are available.
Fixes: QTCREATORBUG-23172
Change-Id: If40304d174469df0c786259e724cbee8de0e2d0e
Reviewed-by: Cristian Adam <cristian.adam@qt.io>