Fix incorrect array bounds with -fgnat-encodings=minimal in DWARF

Fix incorrect array bounds with -fgnat-encodings=minimal in DWARF

This makes add_subscript_info query the get_array_descr_info hook for the
actual information when it is defined.

gcc/
	* cfgexpand.c (expand_gimple_basic_block): Do not inherit a current
	location for the outgoing edges of an empty block.
	* dwarf2out.c (add_subscript_info): Retrieve the bounds and index
	type by means of the get_array_descr_info langhook, if it is set and
	returns true.  Remove obsolete code dealing with unnamed subtypes.
gcc/testsuite/
	* gnat.dg/debug18.adb: New test.