[warning control] New #pragma GCC diagnostic

[DJ Delorie]

I found one regression while testing this.  It turns out that using
reclassification to implement -Werror bypasses the -w (inhibit
warnings) option, because they're not warnings at that point.  It
means that sometimes you get no messages without -Werror, but errors
with -Werror.

So, before I fix this, how do we define the interactions between -w,
-Werror, the -Werror-* options, and the #pragmas?

Does -w inhibit warnings before they're turned into errors?  Before
the pragmas can turn individual warnings into errors?  Can the pragmas
enable warnings which -w would have otherwise inhibited?