[C++11] Do not check __GXX_EXPERIMENTAL_CXX0X__.

Summary: Checking the experimental flag for C++0x is no longer needed.

Differential Revision: http://llvm-reviews.chandlerc.com/D3206

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204964 91177308-0d34-0410-b5e6-96231b3b80d8