The result of symtab expansion is always a primary symtab.

The result of symtab expansion is always a primary symtab.

gdb/ChangeLog:

	* dwarf2read.c (dw2_instantiate_symtab): Add assert.
	(dw2_lookup_symbol): Remove unnecessary test for primary symbol table.
	* psymtab.c (lookup_symbol_aux_psymtabs): Ditto.
	(psymtab_to_symtab): Add comment and assert.
	(map_matching_symbols_psymtab): Remove unnecessary test for
	non-primary symtab.