[RFC diagnostics/fortran] Move gfc_warning (buffered) to the common diagnostics machinery

[Manuel López-Ibáñez <lopezibanez at gmail dot com>]

This is just a RFC so no Changelog yet. It bootstraps and passes the
testsuite. I have three major questions:

* Dodji: Do the common diagnostics part look reasonable? I tried to be
as least invasive as possible. If you have comments or suggestions
they are very welcome.

* Fortran devs: Is this approach acceptable? The main idea is to have
an output_buffer called pp_warning_buffer with the flush_p bit unset
if we are buffering. When printing buffered warnings, use this
output_buffer in the global_dc->printer instead of the (unbuffered
one) used by the *_now variants. In principle this could support
several buffered diagnostics, but Fortran only seems to buffer at most
one.

The ugliest part is how to handle warningcount and werrorcount. I
could handle this in the common machinery in a better way by storing
DK_WERROR in the diagnostic->kind and checking it after printing. This
way we can first decrease both counters, then increase back the one
not changed and store the kind somewhere. then if the diagnostic is
canceled nothing is done, if it is flushed, then increase the
appropriate counter (perhaps calling into the common part for any post
action due to Wfatal-errors or -fmax-errors=).

I can also hide the output_buffer switching inside two helper
functions, but the helper function would need to use either a static
variable or a global one to save and restore the tmp_buffer. I'm not
sure that is better or worse (the current code uses a global pointer
&cur_error_buffer, so perhaps I should have used a similar approach).

* Fortran devs #2: The testsuite is testing that the warning is
eventually printed. However, I'm not sure it is testing when the
warning is buffered and then discarded, is it? If not, how can I
produce such a test?

Thanks,

Manuel.

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