Fix historical #ifdef. Use __cplusplus instead of __GXX_EXPERIMENTAL_CXX0X__ when compiling with clang. No functionality change. Noteto self: It is important to run the regression tests on the copy of the code that you've changed.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@201938 91177308-0d34-0410-b5e6-96231b3b80d8