[clang-proto-fuzzer] Allow user-specified compiler arguments.

Summary:
Arguments can be specified after -ignore_remaining_args=1 to modify
the compiler invocation.  For example, the following command-line
will fuzz LLVM with a custom optimization level and target triple:
  clang-proto-fuzzer CORPUS/ -ignore_remaining_args -O3 \
      -triple arm64-apple-ios9

Reviewers: vitalybuka, kcc

Reviewed By: vitalybuka

Subscribers: aemerson, cfe-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D36882

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311185 91177308-0d34-0410-b5e6-96231b3b80d8

tools/clang-fuzzer/ExampleClangProtoFuzzer.cpp

@@ -20,9 +20,25 @@
 
 #include "src/libfuzzer/libfuzzer_macro.h"
 
+#include <cstring>
+
 using namespace clang_fuzzer;
 
+static std::vector<const char *> CLArgs;
+
+extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv) {
+  CLArgs.push_back("-O2");
+  for (int I = 1; I < *argc; I++) {
+    if (strcmp((*argv)[I], "-ignore_remaining_args=1") == 0) {
+      for (I++; I < *argc; I++)
+        CLArgs.push_back((*argv)[I]);
+      break;
+    }
+  }
+  return 0;
+}
+
 DEFINE_BINARY_PROTO_FUZZER(const Function& input) {
   auto S = FunctionToString(input);
-  HandleCXX(S, {"-O2"});
+  HandleCXX(S, CLArgs);
 }