c++: non-trivial by-value deducing this lambda [PR121500]

c++: non-trivial by-value deducing this lambda [PR121500]

Here the lambda has a by-value capture of non-trivial type, which
in turn makes the closure type non-trivial.  This means its by-value
'this' parameter, which gets deduced to the closure type, becomes an
invisiref parameter, and so when lowering the operator() body we need to
adjust uses of 'this' by adding implicit dereferences.

But the GIMPLE dump for operator() shows that we miss some adjustments:

  bool main()::<lambda(this auto:1)>::operator()<main()::<lambda(this auto:1)> > (struct ._anon_0 & self)
  {
    bool D.3091;
    struct ._anon_0 & self.1;
    struct A a [value-expr: self.__a]; // should be self->__a

    self.1 = self;
    _1 = self.1.__a.n; // should be self.1->__a
    D.3091 = _1 == 42;
    return D.3091;
  }

Apparently this is because cp_genericize_r, which is responsible for the
invisiref use adjustments, never walks DECL_VALUE_EXPR.  This patch makes
us walk it.  For GCC 16, restrict the walking to xobj lambdas.

	PR c++/121500

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_genericize_r): Walk DECL_VALUE_EXPR within
	an xobj lambda.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/explicit-obj-lambda20.C: New test.

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