[Bug c/53072] automatically set Init() only if option was not set in some other way

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53072

--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-04-24 14:43:16 UTC ---
On Tue, 24 Apr 2012, manu at gcc dot gnu.org wrote:

> Say we have: A enables B enables C. Do we actually need to track that C was
> enabled by B or by A? Sorry I cannot imagine what for. 

The idea is that -WB -Wno-B leaves C disabled, but -WB -Wno-A leaves it 
enabled (because -WB is more specific than -Wno-A, it takes precedence 
whatever the ordering of those two options).  If there were any option -WC 
or -Wno-C on the command line, the last such option would apply regardless 
of the -WA / -Wno-A / -WB / -Wno-B options (that much at least is already 
implemented for some option implications).

> For warning options, it is sufficient to track the C was enabled by other
> option different from C. It seems we have most of the infrastructure to do

Even for that, you still need to move away from _set being a boolean, 
since there will be three values (default, explicit -WC / -Wno-C, 
controlled by some other option but not the most specific option).

> this, however, it is unclear how to use this infrastructure to make Init()
> useful again, or implement  PR 53063.

Init() should I think ideally be just for the defaults (with _set 
replacing the present uses of -1 or 2 in Init to mean "not set").