[XRay] clarify error messages when parsing broken traces

Summary:
When there's a mismatch of a function argument being right after the
wrong function, print an offset into the file where that happened, to
ease further debugging.

Reviewers: dberris, eizan, kpw

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42492

llvm-svn: 323758