DebugInfo: Sink constructImportedEntityDIE down into DwarfUnit from DwarfDebug.

It was just calling a bunch of DwarfUnit functions anyway, as can be
seen by the simplification of removing "TheCU" from all the function
calls in the implementation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219103 91177308-0d34-0410-b5e6-96231b3b80d8