[mlir] Tighten access of RewritePattern methods.

In RewritePattern, only expose `matchAndRewrite` as a public function. `match` can be protected (but needs to be protected because we want to call it from an override of `matchAndRewrite`). `rewrite` can be private.

For classes deriving from RewritePattern, all 3 functions can be private.

Side note: I didn't understand the need for the `using RewritePattern::matchAndRewrite` in derived classes, and started poking around. They are gone now, and I think the result is (only very slightly) cleaner.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D92670