[clang-tidy] check_clang_tidy.py: support CHECK-NOTES prefix

Summary:
Currently, there is two configured prefixes: `CHECK-FIXES` and `CHECK-MESSAGES`
`CHECK-MESSAGES` checks that there are no test output lines with `warning:|error:`, which are not explicitly handled in lit tests.
However there does not seem to be a nice way to enforce for all the `note:` to be checked.
This was useful for me when developing D36836.

Reviewers: alexfh, klimek, aaron.ballman, hokein

Reviewed By: alexfh, aaron.ballman

Subscribers: JonasToth, JDevlieghere, xazax.hun, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D36892

llvm-svn: 339437