[XRay] clarify error messages when parsing broken traces
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@323758 91177308-0d34-0410-b5e6-96231b3b80d8

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