c++: -fimplicit-constexpr diagnostic improvement [PR116696]

c++: -fimplicit-constexpr diagnostic improvement [PR116696]

PR116696 expressed surprise that explicit 'constexpr' was needed on one
function; this was because the function isn't 'inline', and
-fimplicit-constexpr doesn't try to promote non-inline functions.  Let's be
more helpful in that situation, and also help trace through functions that
were promoted.

	PR c++/116696

gcc/cp/ChangeLog:

	* constexpr.cc (explain_invalid_constexpr_fn): When
	-fimplicit-constexpr, also explain inline functions, and point out
	non-inline functions.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2478.C: Prune extra diagnostic.
	* g++.dg/ext/fimplicit-constexpr1.C: New test.