[dsymutil] Add DIE selection algorithm.

[dsymutil] Add DIE selection algorithm.

With this commit, llvm-dsymutil learns how to choose which DIEs
it will link in the final output and which ones it won't. This
is based on the 'valid relocation' information that has been
built in the previous commits.

The test only tests that we choose the right 'root DIEs'. The
selection algorithm (and especially the part that walk the
dependencies of a root DIE) lacks a bit test coverage. This
will be much easier to cover when we output actual Dwarf and
thus can use llvm-dwarfdump to verify the structure of the
emitted DIE trees. I'll add more tests then.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229183 91177308-0d34-0410-b5e6-96231b3b80d8