Revert "[C++11] Do not check __GXX_EXPERIMENTAL_CXX0X__."

This reverts commit r204964 because it disabled "= delete", "constexpr"
and "explicit" on GCC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204973 91177308-0d34-0410-b5e6-96231b3b80d8