[clang-tidy] fix check_clang_tidy to forbid mixing of CHECK-NOTES and CHECK-MESSAGES

Summary:
The check_clang_tidy.py script would allow mixing of `CHECK-NOTES` and `CHECK-MESSAGES` but running `FileCheck` for that would implicitly fail, because `CHECK-NOTES` bails out if there is a warning.

That means a clang-tidy test can not mix these constructs to check warnings with `CHECK-MESSAGES` and notes with `CHECK-NOTES`. The script gives now a clear error if that happens.

Reviewers: alexfh, aaron.ballman, lebedev.ri, hokein

Reviewed By: lebedev.ri

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D51381

llvm-svn: 341039