DwarfUnit: Split MDSubroutineType version of constructTypeDIE()

The version of `constructTypeDIE()` for `MDSubroutineType` is unrelated
to (and has different callers than) the `MDCompositeType`.  Split the
two in half.

This simplifies an upcoming patch to delete `DICompositeType`.  There
shouldn't be any real functionality change here.  `createTypeDIE()` is
`cast<>`'ing where it didn't need to before, but that function in turn
is only called for true `MDCompositeType`s.

llvm-svn: 235349