Formatting.
Add a TODO and explain when we can get rid of the isMain field.
Add a field to the compile unit of where we plan on splitting out
the debug info for -gsplit-dwarf so we can encode that location
in the skeleton cu.
Use getSplitDebugFilename when constructing the skeleton cu and
update testcase accordingly to give the correct name to the cu.
Propagate the split dwarf file information through into the backend
and through to the debug info in the module. In order to make the
testcase a bit more efficient allow the filename to go through
compilation for compile and not assemble jobs and turn off the
extract for cases where we don't create an object.