c++: improve verify_constant diagnostic [PR91483]

c++: improve verify_constant diagnostic [PR91483]

When verify_constant complains, it's pretty terse.  Consider

  void test ()
  {
    constexpr int i = 42;
    constexpr const int *p = &i;
  }

where it says "'& i' is not a constant expression".  OK, but why?

With this patch, we say:

b.C:5:28: error: '& i' is not a constant expression
    5 |   constexpr const int *p = &i;
      |                            ^~
b.C:5:28: note: pointer to 'i' is not a constant expression
b.C:4:17: note: address of non-static constexpr variable 'i' may differ on each invocation of the enclosing function; add 'static' to give it a constant address
    4 |   constexpr int i = 42;
      |                 ^
      |                 static

which brings g++ on par with clang++.

	PR c++/91483

gcc/cp/ChangeLog:

	* constexpr.cc (verify_constant_explain_r): New.
	(verify_constant): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/constexpr3.C: New test.