c++: fix truncated diagnostic in C++23 [PR111272]

In C++23, since P2448, a constexpr function F that calls a non-constexpr
function N is OK as long as we don't actually call F in a constexpr
context.  So instead of giving an error in maybe_save_constexpr_fundef,
we only give an error when evaluating the call.  Unfortunately, as shown
in this PR, the diagnostic can be truncated:

z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
   10 |   constexpr Jam() { ft(); }
      |             ^~~

...because what?  With this patch, we say:

z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
   10 |   constexpr Jam() { ft(); }
      |             ^~~
z.C:10:23: error: call to non-'constexpr' function 'int Jam::ft()'
   10 |   constexpr Jam() { ft(); }
      |                     ~~^~
z.C:8:7: note: 'int Jam::ft()' declared here
    8 |   int ft() { return 42; }
      |       ^~

Like maybe_save_constexpr_fundef, explain_invalid_constexpr_fn should
also check the body of a constructor, not just the mem-initializer.

	PR c++/111272

gcc/cp/ChangeLog:

	* constexpr.cc (explain_invalid_constexpr_fn): Also check the body of
	a constructor in C++14 and up.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-diag1.C: New test.

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