gdb: add new function quick_symbol_functions::has_unexpanded_symbols

gdb: add new function quick_symbol_functions::has_unexpanded_symbols

Adds a new function to the quick_symbol_functions API to let us know
if there are any unexpanded symbols.  This functionality is required
by a later commit.  After this commit the functionality is unused, and
untested.

The new function objfile::has_unexpanded_symtabs is added to the
symfile-debug.c file which is a little strange, but this
is (currently) where many of the other objfile::* functions (that call
onto the quick_symbol_functions) are defined, so I'm reluctant to
break this pattern.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* dwarf2/read.c (struct dwarf2_base_index_functions)
	<has_unexpanded_symtabs>: Declare.
	(dwarf2_base_index_functions::has_unexpanded_symtabs): Define new
	function.
	* objfiles.h (struct objfile) <has_unexpanded_symtabs>: Declare.
	* psympriv.h (struct psymbol_functions) <has_unexpanded_symtabs>:
	Declare.
	* psymtab.c (psymbol_functions::has_unexpanded_symtabs): Define
	new function.
	* quick-symbol.h (struct quick_symbol_functions)
	<has_unexpanded_symtabs>: Declare.
	* symfile-debug.c (objfile::has_unexpanded_symtabs): Define new
	function.