Merging r197314:
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r197314 | logan | 2013-12-13 22:45:09 -0800 (Fri, 13 Dec 2013) | 9 lines

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.


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git-svn-id: https://llvm.org/svn/llvm-project/libcxx/branches/release_34@198100 91177308-0d34-0410-b5e6-96231b3b80d8