__attribute__((force)) should not be a storage class

[Josh Triplett <josh@joshtriplett.org>]

Commit 3829c4d8b097776e6b3472290a9fae08a705ab7a ("Don't mix storage
class bits with ctype->modifiers while parsing type") in 2009 separated
storage classes from modifiers; in the process, it changed
__attribute__((force)) from a modifier to a storage class.  I don't
think it makes sense to have force as a storage class, for one critical
reason: storage classes are mutually exclusive.

$ cat /tmp/test.c
static __attribute__((force)) int *p;
static int q = *p;
$ ./sparse /tmp/test.c
/tmp/test.c:1:28: error: multiple storage classes

Given this, I think force should become a modifier again.  Any
objections?

- Josh Triplett