Merging r276473:

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r276473 | vvassilev | 2016-07-22 14:08:24 -0700 (Fri, 22 Jul 2016) | 13 lines

[modules] Teach the ASTWriter to ignore mutations coming from the ASTReader.

Processing update records (and loading a module, in general) might trigger
unexpected calls to the ASTWriter (being a mutation listener). Now we have a
mechanism to suppress those calls to the ASTWriter but notify other possible
mutation listeners.

Fixes https://llvm.org/bugs/show_bug.cgi?id=28332

Patch by Cristina Cristescu and me.

Reviewed by Richard Smith (D21800).

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llvm-svn: 276757