[XRay] clarify error messages when parsing broken traces

[pelikan]

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.

[pelikan]

FWIW, I've been seeing signal handler traces written between ENTER_ARG and ARG_PAYLOAD.

[dberris]

LGTM

[pelikan]

sync

llvm/trunk/lib/XRay/Trace.cpp

for (auto S = Data.drop_front(32); !S.empty(); S = S.drop_front(32)) {
     case 1: { // Arg payload record.
       auto &Record = Records.back();
       // Advance two bytes to avoid padding.
       OffsetPtr += 2;
       int32_t FuncId = RecordExtractor.getSigned(&OffsetPtr, sizeof(int32_t));
       auto TId = RecordExtractor.getU32(&OffsetPtr);
       if (Record.FuncId != FuncId || Record.TId != TId)
         return make_error<StringError>(
-            Twine("Corrupted log, found payload following non-matching "
-                  "function + thread record. Record for ") +
-                Twine(Record.FuncId) + " != " + Twine(FuncId),
+            Twine("Corrupted log, found arg payload following non-matching "
+                  "function + thread record. Record for function ") +
+                Twine(Record.FuncId) + " != " + Twine(FuncId) + "; offset: " +
+                Twine(S.data() - Data.data()),
             std::make_error_code(std::errc::executable_format_error));
       // Advance another four bytes to avoid padding.
       OffsetPtr += 4;
       auto Arg = RecordExtractor.getU64(&OffsetPtr);
       Record.CallArgs.push_back(Arg);
       break;
     }
     default: