Code cleanup: Split dwarf2_ranges_read to a callback
    
    DWARF-5 has .debug_rnglists which is somehow similar to .debug_ranges.
    
    This patch converts dwarf2_ranges_read to dwarf2_ranges_process which can work
    with both DWARF kinds of range lists through a callback.
    
    It also simplifies dwarf2_record_block_ranges which can benefit from it.
    
    gdb/ChangeLog
    2017-02-20  Jan Kratochvil  <jan.kratochvil@redhat.com>
    
    	* dwarf2read.c (dwarf2_ranges_process): New function from
    	dwarf2_ranges_read.
    	(dwarf2_ranges_read, dwarf2_record_block_ranges): Use
    	dwarf2_ranges_process.