Fix a broken test.

Similar to my previous fix, it turns out llvm-pdbdump has been
printing an incorrect value since the beginning of time, but
we didn't know it was incorrect.  Specifically, we were interpreting
a TypeIndex as referencing a type from the TPI stream when it
actually should come from the IPI stream.  So we were printing a
string that looked like a valid string, but was just from the
wrong place.

llvm-svn: 303403