Debug Info: use null instead of "i32 0" in DIBuilder.

For context field of subroutine_type and when creating artificial types.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190270 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r190269 to fix dragonegg failures.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190271 91177308-0d34-0410-b5e6-96231b3b80d8