Factor out helper to determine whether a function is a "member-like

constrained friend".

When a friend declaration has a requires-clause, and either it's a
non-template function or it's a function template whose requires-clause
depends on an enclosing template parameter, it is member-like for the
purpose of redeclaration checking. Specifically, the lexically enclosing
class becomes part of its signature, so it can only be redeclared by
another declaration within the same class. In this change, we call such
functions "member-like constrained friends".

No functional change intended.