[clang-tidy] Add support for writing a check as a Transformer rewrite rule.

This revision introduces an adaptor from Transformer's rewrite rules
(`clang::tooling::RewriteRule`) to `ClangTidyCheck`.  For example, given a
RewriteRule `MyCheckAsRewriteRule`, it lets one define a tidy check as follows:

```
class MyTidyCheck : public TransformerClangTidyCheck {
 public:
  MyTidyCheck(StringRef Name, ClangTidyContext *Context)
      : TransformerClangTidyCheck(MyCheckAsRewriteRule, Name, Context) {}
};
```

Reviewers: aaron.ballman

Subscribers: mgorny, xazax.hun, cfe-commits, ilya-biryukov

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61386

llvm-svn: 361418