DebugInfo: Move getOrCreateSourceID from DwarfDebug to DwarfCompileUnit

We still do a few lookups into the line table mapping in MCContext that
could be factored out into a single lookup (rather than looking it up
once for the table label, once to set the compilation unit, once for
each time we need a file ID, etc... ) but assembly output complicates
that somewhat as we still need a virtual dispatch back to the
MCAsmStreamer in that case.