[clang] [Clang] _default-movable_ should be based on the first declaration (PR #143661)

[Erich Keane via cfe-commits]

erichkeane wrote:

I'm disturbed at how much of these fixes are ending up being `getCanonicalDecl` sprinkled around.  IMO we need to start being better about making the functions we use to 'check' things auto-canonicalizing.

https://github.com/llvm/llvm-project/pull/143661