[clang-tidy] Add fix descriptions to clang-tidy checks.

Summary:
Motivation/Context: in the code review system integrating with clang-tidy,
clang-tidy doesn't provide a human-readable description of the fix. Usually
developers have to preview a code diff (before vs after apply the fix) to
understand what the fix does before applying a fix.

This patch proposes that each clang-tidy check provides a short and
actional fix description that can be shown in the UI, so that users can know
what the fix does without previewing diff.

This patch extends clang-tidy framework to support fix descriptions (will add implementations for
existing checks in the future). Fix descriptions and fixes are emitted via diagnostic::Note (rather than
attaching the main warning diagnostic).

Before this patch:

```
void MyCheck::check(...) {
   ...
   diag(loc, "my check warning") <<  FixtItHint::CreateReplacement(...);
}
```

After:

```
void MyCheck::check(...) {
   ...
   diag(loc, "my check warning"); // Emit a check warning
   diag(loc, "fix description", DiagnosticIDs::Note) << FixtItHint::CreateReplacement(...); // Emit a diagnostic note and a fix
}
```

Reviewers: sammccall, alexfh

Reviewed By: alexfh

Subscribers: MyDeveloperDay, Eugene.Zelenko, aaron.ballman, JonasToth, xazax.hun, jdoerfert, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D59932

llvm-svn: 358576

0 files changed, 0 insertions, 0 deletions