DwarfUnit: Refactor out DW_AT_stmt_list creation into common function for...

DwarfUnit: Refactor out DW_AT_stmt_list creation into common function for fission and non-fission cases

This probably also addresses the FIXME in the fission case regarding
multiple compile units, though I haven't tested that.

This code still confuses me (the literal zero offset makes little sense,
the limitations surrounding asm output I'm not sure about either - but
perhaps we should just always emit one line table? Or should we not rely
on .loc/.file even in assembly so we can produce the same output between
asm and object output?) but this maintains the existing functionality.

llvm-svn: 201441