[clangd] Fix non-idempotent cases of canonicalRenameDecl()

The simplest way to ensure full canonicalization is to canonicalize
recursively in most cases.

This fixes an assertion failure and presumably correctness bugs.

It does show up that D132797's index-based virtual method renames doesn't handle
templates well (the AST behavior is different and IMO better).
We could choose to disable in this case or change the index behavior,
but this patch doesn't do either.

Differential Revision: https://reviews.llvm.org/D133415