Build list of completions as symbol tables are expanded.

Build list of completions as symbol tables are expanded.

This commit makes default_make_symbol_completion_list_break_on build
the list of completions as it expands the necessary symbol tables,
rather than expanding all necessary symbol tables first and then
building the completion lists second.  This allows for the early
termination of symbol table expansion if required.

gdb/ChangeLog:

	* symtab.c (struct add_name_data) <code>: New field.
	Updated comments.
	(add_symtab_completions): New function.
	(symtab_expansion_callback): Likewise.
	(default_make_symbol_completion_list_break_on): Set datum.code.
	Move minimal symbol scan before calling expand_symtabs_matching.
	Scan known primary symtabs for externs and statics before calling
	expand_symtabs_matching.  Pass symtab_expansion_callback as
	expansion_notify argument to expand_symtabs_matching.  Do not scan
	primary symtabs for externs and statics after calling
	expand_symtabs_matching.