[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).

llvm-svn: 276473