c: Fix up pointer types to may_alias structures [PR114493]

c: Fix up pointer types to may_alias structures [PR114493]

The following testcase ICEs in ipa-free-lang, because the
fld_incomplete_type_of
          gcc_assert (TYPE_CANONICAL (t2) != t2
                      && TYPE_CANONICAL (t2) == TYPE_CANONICAL (TREE_TYPE (t)));
assertion doesn't hold.
This is because t is a struct S * type which was created while struct S
was still incomplete and without the may_alias attribute (and TYPE_CANONICAL
of a pointer type is a type created with can_alias_all = false argument),
while later on on the struct definition may_alias attribute was used.
fld_incomplete_type_of then creates an incomplete distinct copy of the
structure (but with the original attributes) but pointers created for it
are because of the "may_alias" attribute TYPE_REF_CAN_ALIAS_ALL, including
their TYPE_CANONICAL, because while that is created with !can_alias_all
argument, we later set it because of the "may_alias" attribute on the
to_type.

This doesn't ICE with C++ since PR70512 fix because the C++ FE sets
TYPE_REF_CAN_ALIAS_ALL on all pointer types to the class type (and its
variants) when the may_alias is added.

The following patch does that in the C FE as well.

2024-06-06  Jakub Jelinek  <jakub@redhat.com>

	PR c/114493
	* c-decl.cc (c_fixup_may_alias): New function.
	(finish_struct): Call it if "may_alias" attribute is
	specified.

	* gcc.dg/pr114493-1.c: New test.
	* gcc.dg/pr114493-2.c: New test.

(cherry picked from commit d5a3c6d43acb8b2211d9fb59d59482d74c010f01)