[Qt] Don't warn about override and final being C++11 extensions

[Qt] Don't warn about override and final being C++11 extensions

Clang will emit a warning when these extensions are used without passing
--std=c++11, but we use feature checking to decide if we have the right
extensions, so we can safely ignore these warnings. The XCode and Windows
project files have the same workaround.

The reason for adding the flag to QMAKE_OBJECTIVE_CFLAGS as well is that
we only have one extra compiler for Objective-C, which is also used for
Objective-C++ sources, so we have to pass the flag, even if it doesn't
make sense for Objective-C.

Reviewed by Simon Hausmann.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105616 268f45cc-cd09-0410-ab3c-d52691b4dbfc