Disable override and final C++ keyword in gcc 4.6.

According to http://gcc.gnu.org/projects/cxx0x.html,
override and final keyword was added in gcc 4.7.  Thus,
we should not use these keywords in gcc 4.6 even when
__GXX_EXPERIMENTAL_CXX0X__ is available.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201679 91177308-0d34-0410-b5e6-96231b3b80d8