Save plain text in the source cache

Save plain text in the source cache

Currently the source cache will only store highlighted text.  However,
there's no reason it could not also store plain text, when styling is
turned off.

This patch makes this change.  This also simplifies the source cache
code somewhat.

gdb/ChangeLog
2019-08-06  Tom Tromey  <tromey@adacore.com>

	* source-cache.c (source_cache::get_plain_source_lines):
	Remove "first_line" and "last_line" parameters.
	(source_cache::get_source_lines): Cache plain text.
	* source-cache.h (class source_cache)
	<get_plain_source_lines>: Update.