[clang] [Clang] Added explanation why `is_constructible` evaluated to false. (PR #143309)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

LGTM. Thanks a lot. 
Will you need us to merge that for you?

https://github.com/llvm/llvm-project/pull/143309