[libFuzzer] add -trace_cmp=1 (guiding mutations based on the observed CMP instructions). This is a reincarnation of the previously deleted -use_traces, but using a different approach for collecting traces. Still a toy, but at least it scales well. Also fix -merge in trace-pc-guard mode

llvm-svn: 284273
diff --git a/llvm/lib/Fuzzer/FuzzerDefs.h b/llvm/lib/Fuzzer/FuzzerDefs.h
index b282a83..99b478f 100644
--- a/llvm/lib/Fuzzer/FuzzerDefs.h
+++ b/llvm/lib/Fuzzer/FuzzerDefs.h
@@ -111,5 +111,11 @@
 int GetPid();
 void SleepSeconds(int Seconds);
 
+
+struct ScopedDoingMyOwnMemmem {
+  ScopedDoingMyOwnMemmem();
+  ~ScopedDoingMyOwnMemmem();
+};
+
 }  // namespace fuzzer
 #endif  // LLVM_FUZZER_DEFS_H