[libFuzzer] provide a plain C interface for custom mutators (experimental)

llvm-svn: 260794
diff --git a/llvm/lib/Fuzzer/test/fuzzer.test b/llvm/lib/Fuzzer/test/fuzzer.test
index aadff79..4638b97 100644
--- a/llvm/lib/Fuzzer/test/fuzzer.test
+++ b/llvm/lib/Fuzzer/test/fuzzer.test
@@ -66,3 +66,7 @@
 RUN: LLVMFuzzer-NthRunCrashTest %t/NthRunCrashTest.in -runs=10
 RUN: not LLVMFuzzer-NthRunCrashTest %t/NthRunCrashTest.in -runs=10000 2>&1 | FileCheck %s
 RUN: rm %t/NthRunCrashTest.in
+
+RUN: not LLVMFuzzer-CustomMutatorTest 2>&1 | FileCheck %s --check-prefix=LLVMFuzzerCustomMutator
+LLVMFuzzerCustomMutator: In LLVMFuzzerCustomMutator
+LLVMFuzzerCustomMutator: BINGO