DebugInfo: Move line table zero-directory-index (compilation dir) handling into MCDwarf

Our handling of compilation directory in DwarfDebug was broken
(incorrectly using the 'last' compilation directory (that of the last
CU in the metadata list) for all function emission in any CU). By moving
this handling down into MCDwarf the issue is fixed as the compilation
dir is tracked correctly per line table.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204089 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204092 91177308-0d34-0410-b5e6-96231b3b80d8