Remove the last mention of LineTablesOnly from DwarfUnit, sinking it into DwarfCompileUnit

This is a useful distinction/invariant/delination to make because
LineTablesOnly mode is never relevant to type units, so it's clear that
we're not doing weird line-tables-only-with-types by making this API
choice.

It also lays the foundations nicely for adding gmlt-like data to fission
skeleton CUs while limiting the effects to CUs and not TUs.

llvm-svn: 221093