A smarter linespec completer

Continuing the theme of the explicit locations patch, this patch gets
rid of the need for quoting function names in linespec TAB completion.
To recap, when you have overloads in your program, and you want to set
a breakpoint in one of them:

 void function(int);  // set breakpoint here.
 void function(long);

 (gdb) b function(i[TAB]
 <all the symbols in the program that start with "i" are uselessly shown...>

This patch gets rid of the need for quoting by switching the linespec
completer to use the custom completion word point mechanism added in
the previous explicit location patch (extending it as needed), to
correctly determine the right completion word point.  In the case
above, we want the completer to figure out that it's completing a
function name that starts with "function(i", and it now does.

We also want the completer to know when it's potentially completing a
source file name, for:

(gdb) break source.[TAB] -> source.c:
(gdb) break source.c:  # Type line number or function name now

And we want it to know to complete label names, which it doesn't today:

(gdb) break function:lab[TAB]

etc., etc.

So what we want is for completion to grok the input string as closely
to how the linespec parser groks it.

With that in mind, the solution suggests itself - make the linespec
completer use the same parsing code as normal linespec parsing.

That's what the patch does.  The old completer is replaced by one that
reuses the actual linespec parser as much as possible.  This (ideally)
eliminate differences between what completion understands and actually
setting breakpoints understands by design.

The completer now offers sensible completion candidates depending on
which component of the linespec is being completed, source filename,
function, line number, expression, and (a new addition), labels.  For
example, when completing the function part, we now show the full name
of the method as completion candidates, instead of showing whatever
comes after what readline considered the word break character:

 (gdb) break klass::method[TAB]
 klass:method1(int)
 klass:method2()

If input is past the function, then we now offer keyword condidates:

  (gdb) b function(int) [TAB]
  if      task    thread

If input is past a keyword, we offer expression completion, which is
different from linespec completion:

  (gdb) b main if 1 + glo[TAB]
  global

(e.g., completes on types, struct data fields, etc.)

As mentioned, this teaches the linespec completer about completing
label symbols too:

  (gdb) b source.c:function:lab[TAB]

A nice convenience is that when completion uniquely matches a source
name, gdb adds the ":" automatically for you:

  (gdb) b filenam[TAB]
  (gdb) b filename.c:  # ':' auto-added, cursor right after it.

It's the little details.  :-)

I worked on this patch in parallel with writing the (big) testcase
added closer to the end of the series, which exercises many many
tricky cases around quoting and whitespace insertion placement.  In
general, I think it now all Just Works.

gdb/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* completer.c (complete_source_filenames): New function.
	(complete_address_and_linespec_locations): New function.
	(location_completer): Use complete_address_and_linespec_locations.
	(completion_tracker::build_completion_result): Honor the tracker's
	request to suppress append.
	* completer.h (completion_tracker::suppress_append_ws)
	(completion_tracker::set_suppress_append_ws): New methods.
	(completion_tracker::m_suppress_append_ws): New field.
	(complete_source_filenames): New declaration.
	* linespec.c (linespec_complete_what): New.
	(struct ls_parser) <complete_what, completion_word,
	completion_quote_char, completion_quote_end, completion_tracker>:
	New fields.
	(string_find_incomplete_keyword_at_end): New.
	(linespec_lexer_lex_string): Record quote char.  If in completion
	mode, don't throw.
	(linespec_lexer_consume_token): Advance the completion word point.
	(linespec_lexer_peek_token): Save/restore completion info.
	(save_stream_and_consume_token): New.
	(set_completion_after_number): New.
	(linespec_parse_basic): Set what to complete next depending on
	token.  Handle function and label completions specially.
	(parse_linespec): Disable objc shortcut in completion mode.  Set
	what to complete next depending on token type.  Skip keyword if in
	completion mode.
	(complete_linespec_component, linespec_complete): New.
	* linespec.h (linespec_complete): Declare.

gdb/testsuite/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* gdb.base/completion.exp: Adjust expected output.
	* gdb.linespec/ls-errs.exp: Don't send tab characters, now that
	the completer works.

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