[CMake] Silence message on multi-configuration generators

The Xcode and Visual Studio generators always log "-- No build type selected, default to Debug". This is because CMake doesn't initialize "CMAKE_CONFIGURATION_TYPES" until the generator's EnableLanguage call gets hit.

The first place EnableLanguage gets hit in our configuration is in the project() call. Since CMAKE_BUILD_TYPE isn't used until after we call project() it is safe to just move this check down a bit.

llvm-svn: 279110