Introducing single for loop into clang_proto_fuzzer

Summary:
Created a new protobuf and protobuf-to-C++ "converter" that wraps the entire C++ code in a single for loop.
  - Slightly changed cxx_proto.proto -> cxx_loop_proto.proto
  - Made some changes to proto_to_cxx files to handle the new kind of protobuf
  - Created ExampleClangLoopProtoFuzzer to test new protobuf and "converter"

Patch by Emmett Neyman

Reviewers: kcc, vitalybuka, morehouse

Reviewed By: vitalybuka, morehouse

Subscribers: mgorny, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D47843

llvm-svn: 334216
diff --git a/clang/tools/clang-fuzzer/cxx_loop_proto.proto b/clang/tools/clang-fuzzer/cxx_loop_proto.proto
new file mode 100644
index 0000000..5dae576
--- /dev/null
+++ b/clang/tools/clang-fuzzer/cxx_loop_proto.proto
@@ -0,0 +1,97 @@
+//===-- cxx_loop_proto.proto - Protobuf description of C++ with for loops -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+///  This file describes a subset of C++ as a protobuf. It is used to
+///  more easily find interesting inputs for fuzzing Clang. This subset
+///  extends the one defined in cxx_proto.proto by adding the option that
+///  a VarRef can use the for loop's counter variable.
+///
+//===----------------------------------------------------------------------===//
+
+
+syntax = "proto2";
+
+message VarRef {
+  required int32 varnum = 1;
+  required bool is_loop_var = 2;
+}
+
+message Lvalue {
+  required VarRef varref = 1;
+}
+
+message Const {
+  required int32 val = 1;
+}
+
+message BinaryOp {
+  enum Op {
+    PLUS = 0;
+    MINUS = 1;
+    MUL = 2;
+    DIV = 3;
+    MOD = 4;
+    XOR = 5;
+    AND = 6;
+    OR = 7;
+    EQ = 8;
+    NE = 9;
+    LE = 10;
+    GE = 11;
+    LT = 12;
+    GT = 13;
+  };
+  required Op op = 1;
+  required Rvalue left = 2;
+  required Rvalue right = 3;
+}
+
+message Rvalue {
+  oneof rvalue_oneof {
+    VarRef varref = 1;
+    Const cons = 2;
+    BinaryOp binop = 3;
+  }
+}
+
+message AssignmentStatement {
+  required Lvalue lvalue = 1;
+  required Rvalue rvalue = 2;
+}
+
+
+message IfElse {
+  required Rvalue cond = 1;
+  required StatementSeq if_body = 2;
+  required StatementSeq else_body = 3;
+}
+
+message While {
+  required Rvalue cond = 1;
+  required StatementSeq body = 2;
+}
+
+message Statement {
+  oneof stmt_oneof {
+    AssignmentStatement assignment = 1;
+    IfElse              ifelse     = 2;
+    While               while_loop = 3;
+  }
+}
+
+message StatementSeq {
+  repeated Statement statements = 1;
+}
+
+message LoopFunction {
+  required StatementSeq statements = 1;
+}
+
+package clang_fuzzer;