Merging r291963:

Merging r291963:
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r291963 | ericwf | 2017-01-13 14:11:40 -0800 (Fri, 13 Jan 2017) | 9 lines

[clang] Emit `diagnose_if` warnings from system headers

Summary: In order for libc++ to meaningfully use `diagnose_if` warnings they need to be emitted from system headers by default. This patch changes the `diagnose_if` warning diagnostic to be shown in system headers.

Reviewers: george.burgess.iv, rsmith, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28703
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_40@293783 91177308-0d34-0410-b5e6-96231b3b80d8