Fix GCC unknown pragma warning in libc++.

We should check defined(__clang__) before the usage of the
clang diagnostic pragmas.

The [-Wswitch] warning in src/future.cpp should be ignored.
As the result, the equivalent GCC pragma is added.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@197314 91177308-0d34-0410-b5e6-96231b3b80d8