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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I like the change, it is a very positive difference.  I'm not quite sure I get the partial-diagnostic/generating the string, and 

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