Revert "Fix historical #ifdef. Use __cplusplus instead of __GXX_EXPERIMENTAL_CXX0X__ when compiling with clang. No functionality change."

This reverts commit r201864, because it breaks our build bots.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@201892 91177308-0d34-0410-b5e6-96231b3b80d8