Previously, the type of explicit object parameters was not considered
for relational operators. This was defined by CWG2586,
<https://cplusplus.github.io/CWG/issues/2586.html>. This fix also means
CWG2547 <https://cplusplus.github.io/CWG/issues/2547.html> is now fully
implemented. Fixes#100329, fixes#104413.
Now start rejecting invalid rvalue reference parameters, which weren't
checked for, and start accepting non-reference explicit object
parameters (like `bool operator==(this C, C) = default;`) which were
previously rejected for the object param not being a reference.
Also start rejecting non-reference explicit object parameters for
defaulted copy/move assign operators (`A& operator=(this A, const A&) =
default;` is invalid but was previously accepted). Fixes#104414.
c7df775440