named warnings & individual warning control

[DJ Delorie]

> I suspect changing such an option (written flag_const_strings; and
> note that for C++, -Wwrite-strings sets warn_write_strings instead,
> which does something different in language-standard terms) in the
> middle of a source file might cause more trouble than changing those
> that do simply condition warnings.

How does this problem change if we disallow enabling messages from the
source?  I.e. we can recategorize errors (from -Werror) as warnings,
or ignore either, but we can't enable a warning or error that wasn't
enabled from the command line?

All the arguments for in-source message control were for disabling
warnings, not enabling them.