ClangTidy: Avoid mixing stdout with stderror when dealing with a large number of files.

Summary:
At Mozilla we are using this tool in order to perform review-time static-analysis, since some patches contain a large number of files we've discovered this issue, where `stderr` gets mixed with `stdout` thus obfuscating our possibility to parse the output.
The patch that we are currently use can be found [here](https://searchfox.org/mozilla-central/source/build/build-clang/clang-tidy-8.patch).

This is just an upstream of the original patch.

Reviewers: JonasToth

Reviewed By: JonasToth

Subscribers: cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D60453

llvm-svn: 357994