[libfuzzer] prune_corpus option for disabling pruning during the load.

Summary:
The option is very useful for testing, plus I intend to measure
its effect on fuzzer effectiveness.

Differential Revision: http://reviews.llvm.org/D21084

llvm-svn: 272035
diff --git a/llvm/lib/Fuzzer/FuzzerInternal.h b/llvm/lib/Fuzzer/FuzzerInternal.h
index 637d2b4..ba4ced5b6 100644
--- a/llvm/lib/Fuzzer/FuzzerInternal.h
+++ b/llvm/lib/Fuzzer/FuzzerInternal.h
@@ -331,6 +331,7 @@
     bool PrintFinalStats = false;
     bool DetectLeaks = true;
     bool TruncateUnits = false;
+    bool PruneCorpus = true;
   };
 
   // Aggregates all available coverage measurements.