gdb: Split global symbol search into separate functions

gdb: Split global symbol search into separate functions

In preparation for the next commit, this commit restructures the code
by splitting global_symbol_searcher::search into separate functions.
There should be no functional changes after this commit.

gdb/ChangeLog:

	* symtab.c (symbol_search::compare_search_syms): Update header
	comment.
	(global_symbol_searcher::is_suitable_msymbol): New function.
	(global_symbol_searcher::expand_symtabs): New function.
	(global_symbol_searcher::add_matching_symbols): New function.
	(global_symbol_searcher::add_matching_msymbols): New function.
	(global_symbol_searcher::search): Move most of the content
	into the new functions above, and call them as needed.
	* symtab.h (global_symbol_searcher) <expand_symtabs>: New member
	function.
	(global_symbol_searcher) <add_matching_symbols>: New member
	function.
	(global_symbol_searcher) <add_matching_msymbols>: New member
	function.
	(global_symbol_searcher) <is_suitable_msymbol>: New member
	function.

Change-Id: I06b26920f35c268f7a38d8203dc2c2813aa501c6