[XRay] Fix enter function tracing for record unwriting

[dberris]

Summary

Before this change, we could run into a situation where we may try to
undo tail exit records after writing metadata records before a function
enter event. This change rectifies that by resetting the tail exit
counter after writing the metadata records.

compiler-rt/lib/xray/xray_fdr_controller.h

bool functionEnter(int32_t FuncId, uint64_t TSC,
     if (finalized() ||
         !prepareBuffer(sizeof(MetadataRecord) + sizeof(FunctionRecord)))
       return returnBuffer();
 
     auto PreambleStatus = recordPreamble(TSC, CPU);
     if (PreambleStatus == PreambleResult::InvalidBuffer)
       return returnBuffer();
 
-    UndoableFunctionEnters = (PreambleStatus == PreambleResult::WroteMetadata)
-                                 ? 1
-                                 : UndoableFunctionEnters + 1;
+    if (PreambleStatus == PreambleResult::WroteMetadata) {
+      UndoableFunctionEnters = 1;
+      UndoableTailExits = 0;
+    } else {
+      ++UndoableFunctionEnters;
+    }
+
     auto Delta = TSC - LatestTSC;
     LastFunctionEntryTSC = TSC;
     LatestTSC = TSC;
     return W.writeFunction(FDRLogWriter::FunctionRecordKind::Enter,
                            mask(FuncId), Delta);
   }
 
   bool functionTailExit(int32_t FuncId, uint64_t TSC,