Move the codebase to use: DWARFCompileUnit -> DWARFUnit

Now the codebase can use the DWARFUnit superclass. It will make it later
seamlessly work also with DWARFPartialUnit for DWZ.

This patch is only a search-and-replace easily undone, nothing interesting
in it.

Differential revision: https://reviews.llvm.org/D42892

llvm-svn: 327810