[libFuzzer] implement strncmp hook for data-flow-guided fuzzing (w/ and w/o dfsan), add a test

llvm-svn: 243611
diff --git a/llvm/lib/Fuzzer/test/fuzzer.test b/llvm/lib/Fuzzer/test/fuzzer.test
index fdabbb1..d6dd3ff 100644
--- a/llvm/lib/Fuzzer/test/fuzzer.test
+++ b/llvm/lib/Fuzzer/test/fuzzer.test
@@ -28,3 +28,6 @@
 RUN: not LLVMFuzzer-MemcmpTest -use_traces=1 -seed=1 -runs=10000    2>&1 | FileCheck %s
 RUN:     LLVMFuzzer-MemcmpTest               -seed=1 -runs=1000000  2>&1 | FileCheck %s --check-prefix=Done1000000
 Done1000000: Done 1000000 runs in
+
+RUN: not LLVMFuzzer-StrncmpTest -use_traces=1 -seed=1 -runs=10000    2>&1 | FileCheck %s
+RUN:     LLVMFuzzer-StrncmpTest               -seed=1 -runs=1000000  2>&1 | FileCheck %s --check-prefix=Done1000000