[clang] [clang-tools-extra] [clang][nfc] Define ConstRecursiveASTVisitor twin of RecursiveASTVisitor (PR #160065)

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Sirraide wrote:

> Yes, I have converted a lot of uses of RAV to use DRAV (see #115144) and there are some more left that I still need to migrate; I should be able to get back to that soon.

I finally got around to doing this now: #160290. After this change, there should only be a few visitors left that use RAV instead of DRAV.

https://github.com/llvm/llvm-project/pull/160065