Code cleanup: dwarf2_get_pc_bounds: -1/0/+1 -> enum

Code cleanup: dwarf2_get_pc_bounds: -1/0/+1 -> enum

Make the code (maybe) more readable + primarily prepare it for [patch 2/2]
enum extension.

This change should have no code change impact.

gdb/ChangeLog
2016-05-30  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Code cleanup: dwarf2_get_pc_bounds: -1/0/+1 -> enum
	* dwarf2read.c (enum pc_bounds_kind) New.
	(dwarf2_get_pc_bounds): Use it in the declaration.
	(process_psymtab_comp_unit_reader): Adjust caller.  Rename has_pc_info
	to cu_bounds_kind.
	(read_func_scope, read_lexical_block_scope, read_call_site_scope):
	Adjust callers.
	(dwarf2_get_pc_bounds): Use enum pc_bounds_kind in the definition.
	(dwarf2_get_subprogram_pc_bounds, get_scope_pc_bounds): Adjust callers.