The instantiation used to deduce the return types of template functions with placeholders in their declared return type happens outside the immediate context of any other template instantiation. Notably, that means the body of such an instantiation isn't an unevaluated context.

Switching to concrete return types will:

1. Avoid return type deduction, correcting the problem described in DevCom-10745303, and
2. Correct the ODR problem described in Potential ODR violation linking C++14 with C++17+ TUs that use container emplace functions #4962.

Fixes DevCom-10745303 / VSO-2252142 / AB#2252142