gdb: make struct output_source_filename_data more C++ like

gdb: make struct output_source_filename_data more C++ like

In a future commit I'm going to be making some changes to the 'info
sources' command.  While looking at the code I noticed that things
could be improved by making struct output_source_filename_data more
C++ like (private member variables, and more member functions).
That's what this commit does.

The 'info sources' filename filtering is split out into a separate
class in this commit.  In a future commit this new filter
class (info_sources_filter) will move into the header file and be used
from the MI code.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* symtab.c (struct info_sources_filter): New.
	(info_sources_filter::info_sources_filter): New function.
	(info_sources_filter::matches): New function.
	(info_sources_filter::print): New function.
	(struct filename_partial_match_opts): Moved to later in the file
	and update the comment.
	(struct output_source_filename_data)
	<output_source_filename_data>: New constructor.  <regexp>: Delete,
	this is now in info_sources_filter.  <c_regexp>: Delete, this is
	now in info_sources_filter.  <reset_output>: New member function.
	<filename_seen_cache>: Rename to m_filename_seen_cache, change
	from being a pointer, to being an actual object.  <first>: Rename
	to m_first.  <print_header>: New member function. <partial_match>:
	Delete.
	(output_source_filename_data::output): Update now
	m_filename_seen_cache is no longer a pointer, and for other member
	variable name changes. Add a header comment.
	(print_info_sources_header): Renamed to...
	(output_source_filename_data::print_header): ...this.  Update now
	it's a member function and to take account of member variable
	renaming.
	(info_sources_command): Add a header comment, delete stack local
	filename_seen_cache, initialization of output_source_filename_data
	is now done by the constructor.  Call print_header member function
	instead of print_info_sources_header, call reset_output member
	function instead of manually performing the reset.