Fix regression: expression completer and scope operator (PR gdb/22584)

Fix regression: expression completer and scope operator (PR gdb/22584)

I noticed this regression in the expression completer:

 "(gdb) p std::[TAB]" => "(gdb) p std::std::"

obviously we should have not completed to "std::std::".

The problem is that in the earlier big completer rework, I missed
taking into account the fact that with expressions, the completion
word point is not always at the start of the symbol name (it is with
linespecs).

The fix is to run the common prefix / LCD string (what readline uses
to expand the input line) through make_completion_match_str too.

New testcase included, exercising both TAB completion and the complete
command.

gdb/ChangeLog:
2017-12-13  Pedro Alves  <palves@redhat.com>

	* completer.c (completion_tracker::maybe_add_completion): New
	'text' and 'word' parameters.  Use make_completion_match_str.
	(completion_tracker::add_completion): New 'text' and 'word'
	parameters.  Pass down.
	(completion_tracker::recompute_lowest_common_denominator): Change
	parameter type to gdb::unique_xmalloc_ptr rval ref.  Adjust.
	* completer.h (completion_tracker::add_completion): New 'text' and
	'word' parameters.
	(completion_tracker::recompute_lowest_common_denominator): Change
	parameter type to gdb::unique_xmalloc_ptr rval ref.
	(completion_tracker::recompute_lowest_common_denominator): Change
	parameter type to gdb::unique_xmalloc_ptr rval ref.
	* symtab.c (completion_list_add_name): Pass down 'text' and 'word'
	as well.

gdb/testsuite/ChangeLog:
2017-12-13  Pedro Alves  <palves@redhat.com>

	* gdb.cp/cpcompletion.exp: Load completion-support.exp.
	("expression with namespace"): New set of tests.
	* gdb.cp/pr9594.cc (Test_NS::foo, Test_NS::bar)
	(Nested::Test_NS::qux): New.
	* lib/completion-support.exp (test_gdb_complete_cmd_multiple): Add
	defaults to 'start_quote_char' and 'end_quote_char' parameters.