Carbon fuzzing 2/3: added logic for converting carbon.proto to a Carbon source string. (#1155)

* Carbon fuzzing 2/3: added logic for converting carbon.proto to a Carbon source string.

* adjusted to proto changes in proto PR

* addressed todos

* removed unnecessary has_xx() calls

* minor fix

* sync from part 1

* Update executable_semantics/fuzzing/proto_to_carbon_test.cpp

Co-authored-by: Geoff Romer <gromer@google.com>

* review comments

* Update common/fuzzing/proto_to_carbon.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update common/fuzzing/proto_to_carbon.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* code review

* code review

* Update common/fuzzing/proto_to_carbon.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* code review

* merged + added support for type_params in class

* use <> includes for protobuf

* review comments

* code review comments

Co-authored-by: Geoff Romer <gromer@google.com>
Co-authored-by: Jon Meow <jperkins@google.com>

executable_semantics/fuzzing/BUILD

@@ -35,3 +35,25 @@ cc_test(
         "@llvm-project//llvm:Support",
     ],
 )
+
+cc_test(
+    name = "proto_to_carbon_test",
+    srcs = ["proto_to_carbon_test.cpp"],
+    args = [
+        "$(locations //executable_semantics:standard_libraries)",
+        "$(locations //executable_semantics/testdata:carbon_files)",
+    ],
+    data = [
+        "//executable_semantics:standard_libraries",
+        "//executable_semantics/testdata:carbon_files",
+    ],
+    deps = [
+        ":ast_to_proto_lib",
+        "//common/fuzzing:carbon_cc_proto",
+        "//common/fuzzing:proto_to_carbon_lib",
+        "//executable_semantics/syntax",
+        "@com_google_googletest//:gtest",
+        "@com_google_protobuf//:protobuf_headers",
+        "@llvm-project//llvm:Support",
+    ],
+)