Reorder/reindent dw2_expand_symtabs_matching & friends

Reorder/reindent dw2_expand_symtabs_matching & friends

The previous patch had added dw2_expand_symtabs_matching_symbol and
dw2_expand_marked_cus forward declarations and did not reindent
dw2_expand_marked_cus to avoid moving the code around while changing
it at the same time.

gdb/ChangeLog:
2017-11-08  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (dw2_expand_marked_cus)
	(dw2_expand_symtabs_matching_symbol): Remove forward declarations.
	(dw2_expand_symtabs_matching): Move further below.
	(dw2_expand_marked_cus): Reindent.