c++: Fix up ubsan false positives on references [PR95693]

c++: Fix up ubsan false positives on references [PR95693]

Alex' 2 years old change to build_zero_init_1 to return NULL pointer with
reference type for references breaks the sanitizers, the assignment of NULL
to a reference typed member is then instrumented before it is overwritten
with a non-NULL address later on.
That change has been done to fix error recovery ICE during
process_init_constructor_record, where we:
          if (TYPE_REF_P (fldtype))
            {
              if (complain & tf_error)
                error ("member %qD is uninitialized reference", field);
              else
                return PICFLAG_ERRONEOUS;
            }
a few lines earlier, but then continue and ICE when build_zero_init returns
NULL.

The following patch reverts the build_zero_init_1 change and instead creates
the NULL with reference type constants during the error recovery.

The pr84593.C testcase Alex' change was fixing still works as before.

2021-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/95693
	* init.c (build_zero_init_1): Revert the 2018-03-06 change to
	return build_zero_cst for reference types.
	* typeck2.c (process_init_constructor_record): Instead call
	build_zero_cst here during error recovery instead of build_zero_init.

	* g++.dg/ubsan/pr95693.C: New test.

(cherry picked from commit e5750f847158e7f9bdab770fd9c5fff58c5074d3)