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.