c++: Add test for c++/93807

This PR was initially accepts-invalid, but I think it's actually valid
C++20 code.  My reasoning is that in C++20 we no longer require the
declaration of operator== (#if-defed in the test), because C++20's
[temp.names]/2 says "A name is also considered to refer to a template
if it is an unqualified-id followed by a < and name lookup either finds
one or more functions or finds nothing." so when we're parsing

  constexpr friend bool operator==<T>(T lhs, const Foo& rhs);

we treat "operator==" as a template name, because name lookup of
"operator==" found nothing and we have an operator-function-id, which is
an unqualified-id, and it's followed by a <.  So the declaration isn't
needed to treat "operator==<T>" as a template-id.

	PR c++/93807
	* g++.dg/cpp2a/fn-template20.C: New test.

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