diagnostic.c: add print_option_information

diagnostic.c: add print_option_information

This patch simplifies diagnostic_report_diagnostic by moving
option-printing to a new subroutine.

Doing so required a slight rewrite.  In both the old and new
code, context->option_name returns a malloc-ed string.
The old behavior was to then use ACONCAT to manipulate the
format_spec, appending the option metadata.
ACONCAT calcs the buffer size, then uses alloca, and then copies the
data to the on-stack buffer.

Given the alloca, this needs rewriting when moving the printing to
a subroutine.  In the new version, the metadata is simply printed
using pp_* calls (so it's hitting the obstack within the
pretty_printer).

This means we can get rid of the save/restore of format_spec: I don't
believe anything else in the code modifies it.

It also seems inherently simpler; it seems odd to me to be
appending metadata to the formatting string, rather than simply
printing the metadata after the formatted string is printed
(the old code also assumed that no option name contained a '%').

No functional change intended.

gcc/ChangeLog:
	* diagnostic.c (diagnostic_report_diagnostic): Eliminate
	save/restor of format_spec.  Move option-printing code to...
	(print_option_information): ...this new function, and
	reimplement by simply printing to the pretty_printer,
	rather than appending to the format string.

From-SVN: r247661