run-clang-tidy: forward clang-tidy exit status

Exit with a non-zero value in case any of the underlying clang-tidy
invocations exit with a non-zero value.

This is useful in case WarningsAsErrors is enabled for some of the
checks: if any of those checks find something, the exit status now
reflects that.

Also add the ability to use run-clang-tidy.py via lit, and assert that
the exit code is not 0 when modernize-use-auto is triggered
intentionally.

Reviewers: alexfh, aaron.ballman

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D44366

llvm-svn: 327854