[Bug c++/65575] [c++-concepts] Parse error for requires clause on functions that return a reference type

[andrew.n.sutton at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65575

--- Comment #4 from Andrew Sutton <andrew.n.sutton at gmail dot com> ---
Apparently I do not understand declarators. The attached patch searches through
the declarator structure to filter out declarator structures to which a
requires-clause cannot be attached.

I updated the pr665575.C test to include more examples (positive and negative),
but that's not reflected in the attached patch. I haven't been able to test as
thoroughly as I'd like because I'm at the airport, but this seems like the
right thing to do.