do not ignore attribute 'noreturn'...

do not ignore attribute 'noreturn'...

Hello,

enclosed is a simple patch adding support for attribute 'noreturn' to the
parser. The enhancement makes it possible to optimize walk through CFG and
thus help us to fight with the state explosion. The benefit is demonstrated
on a simple real-world example.

Generated CFG before patch:
http://dudka.cz/devel/html/slsparse-before/slplug.c-handle_stmt_assign.svg

Generated CFG after patch:
http://dudka.cz/devel/html/slsparse-after/slplug.c-handle_stmt_assign.svg

It's one of the key features I am currently missing in SPARSE in contrast
to gcc used as parser. Thanks in advance for considering it!

Signed-off-by: Kamil Dudka <kdudka@redhat.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>