[clang] [clang] Add clang::default_constructed_after_move attribute (PR #178432)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Given that there are a bunch of design questions that comes to mind here, notably whether the attribute is on the declaration of the class or the constructor, the impact of move assignment operators, constraints etc and the fact we need more illustration of usage, I would like to see an RFC for that before moving forward with the PR

@AaronBallman @erichkeane

https://github.com/llvm/llvm-project/pull/178432