[libFuzzer] add an experimental flag -focus_function: libFuzzer will try to focus on inputs that trigger that function

llvm-svn: 332554
diff --git a/compiler-rt/test/fuzzer/OnlySomeBytesTest.cpp b/compiler-rt/test/fuzzer/OnlySomeBytesTest.cpp
index 05793f0..3873b71 100644
--- a/compiler-rt/test/fuzzer/OnlySomeBytesTest.cpp
+++ b/compiler-rt/test/fuzzer/OnlySomeBytesTest.cpp
@@ -12,6 +12,7 @@
 const size_t N = 2048;
 typedef const uint8_t *IN;
 
+extern "C" {
 __attribute__((noinline)) void bad() {
   fprintf(stderr, "BINGO\n");
   abort();
@@ -27,6 +28,8 @@
 __attribute__((noinline)) void fB(IN in) { if (in[1] == 'B') fC(in); }
 __attribute__((noinline)) void fA(IN in) { if (in[0] == 'A') fB(in); }
 
+} // extern "C"
+
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   if (Size < N) return 0;
   fA((IN)Data);