Set list_in_scope later in DWARF reader

Set list_in_scope later in DWARF reader

Currently the DWARF reader sets the list_in_scope member of the CU
when first starting to process symbols.  Future changes will make this
assert -- code will not be able to refer to these lists until after
start_symtab has been called.  This patch prepares for the problem by
arranging to initialize list_in_scope in dwarf2_start_symtab.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (process_full_comp_unit): Do not set list_in_scope.
	(process_full_type_unit): Likewise.
	(dwarf2_start_symtab): Set list_in_scope.