[DWARF parser] Refactor fetching DIE address ranges.

Add a helper method to get address ranges specified in a DIE
(either by DW_AT_low_pc/DW_AT_high_pc, or by DW_AT_ranges). Use it
to untangle and simplify the code.

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206624 91177308-0d34-0410-b5e6-96231b3b80d8