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

[Caslyn Tonelli via cfe-commits]

Caslyn wrote:

Hi @egorshamshura and @cor3ntin, we are seeing the test failures mentioned in the previous comment in our downstream builders as well. 

If a forward-fix can't be produced quickly, could you please revert this change while investigating?

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