Clean up dwarf writer, part 1. This eliminated the horrible recursive getGlobalVariab...

Clean up dwarf writer, part 1. This eliminated the horrible recursive getGlobalVariablesUsing and replaced it something readable. It eliminated use of slow UniqueVector and replaced it with StringMap, SmallVector, and DenseMap, etc. It also fixed some non-deterministic behavior.

This is a very minor compile time win.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65438 91177308-0d34-0410-b5e6-96231b3b80d8