move flag_debug_compare out of diagnostics.c

[Gabriel Dos Reis]

Manuel López-Ibáñez <lopezibanez@gmail.com> writes:

| PING: either the original patch
| 
| http://gcc.gnu.org/ml/gcc-patches/2010-02/msg00819.html
| 
| or a new one using inhibit_notes + inhibit_warnings.

The second version is OK.