Add -funconstrained-commons to work around PR/69368 (and others) in SPEC2006

Add -funconstrained-commons to work around PR/69368 (and others) in SPEC2006

gcc/:

        * common.opt (funconstrained-commons, flag_unconstrained_commons): New.
        * tree.c (array_at_struct_end_p): Do not limit to size of decl for
        DECL_COMMONS if flag_unconstrained_commons is set.
        * tree-dfa.c (get_ref_base_and_extent): Likewise.
        * doc/invoke.texi (Optimize Options): Add -funconstrained-commons.
        (funconstrained-commons): Document.

gcc/testsuite:

	* gfortran.dg/unconstrained_commons.f: New.

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>
From-SVN: r234106