vla: support for DW_AT_count

vla: support for DW_AT_count

This patch adds support for DW_AT_count as requested in the code review:

  https://sourceware.org/ml/gdb-patches/2013-11/msg00200.html

gdb/ChangeLog:

	* dwarf2read.c (read_subrange_type): Convert DW_AT_count to a dynamic
	property and store it as the high bound and flag the range accordingly.
	* gdbtypes.c (resolve_dynamic_bounds): If range is flagged as
	RANGE_UPPER_BOUND_IS_COUNT assign low + high - 1 as the new high bound.
	* gdbtypes.h (enum range_flags): New enum.
	(struct range_bounds): Add flags member.