[clang] [Clang][RFC] Do not eat SFINAE diagnostics for explicit template arguments (PR #139066)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

@erichkeane @mizvekov any objection to move forward with this as-is? It is an improvement over the status quo and changes to diagnostics have not materialized / are a lot more involved

(Happy birthday to that PR :cake: )

https://github.com/llvm/llvm-project/pull/139066