[Dsymutil][Debuginfo][NFC] Refactor dsymutil to separate DWARF optimizing part 2.

That patch is extracted from the D70709. It moves CompileUnit, DeclContext
into llvm/DebugInfo/DWARF. It also adds new file DWARFOptimizer with
AddressesMap class. AddressesMap generalizes functionality
from RelocationManager.

Differential Revision: https://reviews.llvm.org/D71271