Prevent modernize-use-auto from emitting a warning when 'auto' was already being used.

Summary: This fixes https://llvm.org/bugs/show_bug.cgi?id=25082 .

Reviewers: bkramer, klimek

Subscribers: cfe-commits, alexfh

Differential Revision: http://reviews.llvm.org/D13504

llvm-svn: 250284