[concepts] Implement dcl.decl.general p4: No constraints on non-template funcs

The standard says:
The optional requires-clause ([temp.pre]) in an init-declarator or
member-declarator shall be present only if the declarator declares a
templated function ([dcl.fct]).

This implements that limitation, and updates the tests to the best of my
ability to capture the intent of the original checks.

Differential Revision: https://reviews.llvm.org/D125711