do not emit -Wunused-macros warnings in -frewrite-includes mode (PR15614)

-frewrite-includes calls PP.SetMacroExpansionOnlyInDirectives() to avoid
macro expansions that are useless in that mode, but this can lead
to -Wunused-macros false positives. As -frewrite-includes does not emit
normal warnings, block -Wunused-macros too.

Differential Revision: https://reviews.llvm.org/D65371

llvm-svn: 372026