Loosen one check from r233446: as long as `DIBuilder` requires a
non-null type for every subprogram, we should allow a null type array.
Also add tests for the rest of `MDSubroutineType`, which were somehow
missing.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233468 91177308-0d34-0410-b5e6-96231b3b80d8