[clang-tidy] New check bugprone-unused-return-value

Summary:
Detects function calls where the return value is unused.

Checked functions can be configured.

Reviewers: alexfh, aaron.ballman, ilya-biryukov, hokein

Reviewed By: alexfh, aaron.ballman

Subscribers: hintonda, JonasToth, Eugene.Zelenko, mgorny, xazax.hun, cfe-commits

Tags: #clang-tools-extra

Patch by Kalle Huttunen!

Differential Revision: https://reviews.llvm.org/D41655

llvm-svn: 327833