Define Q_CC_CLANG to be the version of upstream Clang that's in use

We map the Apple Clang versions to upstream, so that we have one
define to compare against.

Fixes build break on iOS due to qbasicatomic.h not defining
QT_BASIC_ATOMIC_HAS_CONSTRUCTORS on Apple Clang versions, which
is needed after 1e9db9f5



Change-Id: I17493c0187c20abc5d22e71944d62bfd16afbad2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>