[dsymutil] Move abstractions into separate files (NFC)

[dsymutil] Move abstractions into separate files (NFC)

This patch splits off some abstractions used by dsymutil's dwarf linker
and moves them into separate header and implementation files. This
almost halves the number of LOC in DwarfLinker.cpp and makes it a lot
easier to understand what functionality lives where.

Differential revision: https://reviews.llvm.org/D48647

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335749 91177308-0d34-0410-b5e6-96231b3b80d8