Build fix for gcc builtin

The __sync_add_and_fetch() builtin parameter is volatile but clang has
'different' type checking and ends up accepting this code.

Undo the C++ cast from r198505 to get libc++/LLVM building with g++ while this
is investigated.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@199494 91177308-0d34-0410-b5e6-96231b3b80d8