DebugInfo: Refactor CompileUnit into a Unit baseclass and CompileUnit/TypeUnit derived classes.

Header/cpp file rename to follow immediately - just splitting out the
commits for ease of review/reading to demonstrate that the renaming
changes are entirely mechanical.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196139 91177308-0d34-0410-b5e6-96231b3b80d8
DebugInfo: Rename DwarfCompileUnit.* to DwarfUnit.* to match their contents.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196140 91177308-0d34-0410-b5e6-96231b3b80d8