[dsymutil] Add DwarfLinker class.

It's an empty shell for now. It's main method just opens the debug
map objects and parses their Dwarf info. Test that we at least do
that correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227337 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed