[C++11] Remove use of LLVM_HAS_RVALUE_REFERENCES from Clang. This macro
is now always 1, as we're requiring C++11 now!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202584 91177308-0d34-0410-b5e6-96231b3b80d8