c++: constexpr base-to-derived conversion with offset 0 [PR103879]

c++: constexpr base-to-derived conversion with offset 0 [PR103879]

r12-136 made us canonicalize an object/offset pair with negative offset
into one with a nonnegative offset, by iteratively absorbing the
innermost component into the offset and stopping as soon as the offset
becomes nonnegative.

This patch strengthens this transformation by making it keep on absorbing
even if the offset is already 0 as long as the innermost component is at
position 0 (and thus absorbing doesn't change the offset).  This lets us
accept the two constexpr testcases below, which we'd previously reject
essentially because cxx_fold_indirect_ref would be unable to resolve
*(B*)&b.D123 (where D123 is the base A subobject at position 0) to just b.

	PR c++/103879

gcc/cp/ChangeLog:

	* constexpr.c (cxx_fold_indirect_ref): Split out object/offset
	canonicalization step into a local lambda.  Strengthen it to
	absorb more components at position 0.  Use it before both calls
	to cxx_fold_indirect_ref_1.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-base2.C: New test.
	* g++.dg/cpp1y/constexpr-base2a.C: New test.