[clang] [Clang] Fix constant eval of assignment operators with an explicit object parameter (PR #142964)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I'm not particularly comfortable in ExprConstant, so perhaps we should wait for a smarter reviewer, but I didn't see anything that looked wrong to me.

https://github.com/llvm/llvm-project/pull/142964