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/ast_to_proto_test.cpp

@@ -5,6 +5,7 @@
 #include "executable_semantics/fuzzing/ast_to_proto.h"
 
 #include <gmock/gmock.h>
+#include <google/protobuf/descriptor.h>
 #include <gtest/gtest.h>
 
 #include <filesystem>
@@ -13,7 +14,6 @@
 #include <variant>
 
 #include "executable_semantics/syntax/parse.h"
-#include "google/protobuf/descriptor.h"
 #include "llvm/Support/Error.h"
 
 namespace Carbon::Testing {