[clang-tidy] Add -quiet option to suppress extra output

Summary:
This new flag instructs clang-tidy to not output anything
except for errors and warnings.  This makes it easier to
script clang-tidy to run as part of external build systems.

Reviewers: bkramer, alexfh, klimek

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D29661

llvm-svn: 294607