[DWARF] - Refactoring: localize handling of relocations in a single place.

This is splitted from D32228,
currently DWARF parsers code has few places that applied relocations values manually.
These places has similar duplicated code. Patch introduces separate method that can be
used to obtain relocated value. That helps to reduce code and simplifies things.

Differential revision: https://reviews.llvm.org/D32284

llvm-svn: 300956