pretty-print: split up pretty_printer::format into subroutines

The body of pretty_printer::format is almost 500 lines long,
mostly comprising two distinct phases.

This patch splits it up so that there are explicit subroutines
for the two different phases, reducing the scope of various
locals, and making it easier to e.g. put a breakpoint on phase 2.

No functional change intended.

gcc/ChangeLog:
	* pretty-print-markup.h (pp_markup::context::context): Drop
	params "buf" and "chunk_idx", initializing m_buf from pp.
	(pp_markup::context::m_chunk_idx): Drop field.
	* pretty-print.cc (pretty_printer::format): Convert param
	from a text_info * to a text_info &.  Split out phase 1
	and phase 2 into subroutines...
	(format_phase_1): New, from pretty_printer::format.
	(format_phase_2): Likewise.
	* pretty-print.h (pretty_printer::format): Convert param
	from a text_info * to a text_info &.
	(pp_format): Update for above change.  Assert that text_info is
	non-null.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>