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

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

It feels like having `isDefaultedOnDeclaration()` would be tremendously more readable and express the intent rather than doing "the dance" when necessary. 

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