Improve efficiency of mutator by allowing Copy/Clone only in CrossOver

That's very libFuzzer specific tweek
libFuzzer calls LLVMFuzzerCustomMutator and LLVMFuzzerCustomCrossOver
with same probability. However LLVMFuzzerCustomCrossOver is
just a variation of Copy/Clone. So before the patch all mutator activity
was more skewed toward Copy/Clone.

This results in 2x increases in new feature discovery per iteration.

Author: vitalybuka

src/mutator.cc

@@ -684,6 +684,8 @@ bool Mutator::MutateImpl(const ConstMessages& sources, const Messages& messages,
     mutations[static_cast<size_t>(Mutation::Delete)] = true;
   } else {
     mutations.set();
+    mutations[static_cast<size_t>(Mutation::Copy)] = false;
+    mutations[static_cast<size_t>(Mutation::Clone)] = false;
   }
   while (mutations.any()) {
     MutationSampler mutation(keep_initialized_, mutations, &random_);

src/mutator_test.cc

@@ -555,7 +555,7 @@ void MutatorFieldTest::TestCopyField() {
     to.add_repeated_msg()->CopyFrom(*m1_);
     to.add_repeated_msg()->CopyFrom(*m1_);
     to.mutable_repeated_msg(1)->add_repeated_string("repeated_string");
-    EXPECT_TRUE(Mutate(from, to));
+    EXPECT_TRUE(CrossOver(from, from, to));
   }
 }