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
llvm-svn: 291963
Eric Fiselier