include: Add macro to ignore -Wunused-but-set-variable

"-Wunused-but-set-variable" warning option can be helpful to track variables
that are written but not read thereafter.  But it can be harmful if some of
the code is auto-generated and we have no ways to deal with it.

The particular example is Bison-generated code.

The new DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE macro can be helpful on
such cases. A typical use of this macro is to place this macro before the
end of user prologues on Bison (.y) files.

include/ChangeLog:

    * diagnostics.h (DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE): New.