c++: poor diag w/ non-constexpr dtor called from constexpr ctor

When diagnosing a non-constexpr constructor call during constexpr
evaluation, explain_invalid_constexpr_fn was passing the genericized
body to require_potential_constant_expression rather than the saved
non-genericized one.

This meant for the below testcase (reduced from PR libstdc++/119282)
in which B::B() is deemed non-constexpr due to the local variable having
a non-constexpr destructor we would then issue the cryptic diagnostic:

constexpr-nonlit19.C:17:16: error: non-constant condition for static assertion
   17 | static_assert(f());
      |               ~^~
constexpr-nonlit19.C:17:16:   in ‘constexpr’ expansion of ‘f()’
constexpr-nonlit19.C:13:5: error: ‘constexpr B::B()’ called in a constant expression
   13 |   B b;
      |     ^
constexpr-nonlit19.C:6:13: note: ‘constexpr B::B()’ is not usable as a ‘constexpr’ function because:
    6 |   constexpr B() {
      |             ^
constexpr-nonlit19.C:8:5: error: ‘goto’ is not a constant expression
    8 |     for (int i = 0; i < 10; i++) { }
      |     ^~~

This patch makes us pass the non-genericized body to
require_potential_constant_expression, and so we now emit:

...
constexpr-nonlit19.C:6:13: note: ‘constexpr B::B()’ is not usable as a ‘constexpr’ function because:
    6 |   constexpr B() {
      |             ^
constexpr-nonlit19.C:9:3: error: call to non-‘constexpr’ function ‘A::~A()’
    9 |   }
      |   ^
constexpr-nonlit19.C:3:12: note: ‘A::~A()’ declared here
    3 | struct A { ~A() { } };
      |            ^

gcc/cp/ChangeLog:

	* constexpr.cc (explain_invalid_constexpr_fn): In the
	DECL_CONSTRUCTOR_P branch pass the non-genericized body to
	require_potential_constant_expression.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/constexpr-nonlit19.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

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