Print stats on displayed and ignored warnings.

Summary:
Also displays a hint to use -header-filter='.*' in case any warnings
are in non-user code. This will help discoverability of this option.

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3621

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@208174 91177308-0d34-0410-b5e6-96231b3b80d8