[clang-tidy] add reproducer for PR39949 into test-suite

Summary:
The underlying issue is fixed in https://reviews.llvm.org/D56444
and this test ensures the issue does not creep back into our
code-base.

Reviewers: alexfh, aaron.ballman, hokein, hwright

Reviewed By: aaron.ballman

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D56918

llvm-svn: 351569