[DWARF] - Refactoring of DWARFContextInMemory implementation.

This change is basically relative to D31136, where I initially wanted to
implement some relocations handling optimization which shows it can give
significant boost. Though even without any caching algorithm looks
code can have some cleanup at first.

Refactoring separates out code for taking symbol address, used in relocations
computation.

Differential revision: https://reviews.llvm.org/D31747

llvm-svn: 300039