Improve requirement clause limitation on non templated function

Improve requirement clause limitation on non templated function

The current implementation 6da3d66f03f9162ef341cc67218be40e22fe9808
got a few things wrong, particularly that a template, or  definition
or member in a templated entity is required to be allowed to have a
trailing requires clause.

This patch corrects this, as reproted by #61748

Fixes: #61748

Differential Revision: https://reviews.llvm.org/D147070