The code using the StmtPrinterHelper object failed to account for a null object in many cases, which could have led to crashes were it ever to be null. Now passing the object by reference instead of by pointer because it is never null in practice. No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195043 91177308-0d34-0410-b5e6-96231b3b80d8