Carbon fuzzing 1/3: added a proto for representing Carbon AST + ast_to_proto library (#1128)

* initial fuzzer proto

* visibility change

* use newer protocol buffer version which has the defs.bzl bug fixed

* Adjusted fix_cc_deps to work with protobuf external repo

* explicitly load rules_cc to avoid a frozenset bug in the version loaded by protobuf

* use explit deps, use llvm's zlib

* restored cxx settings

* deps change

* Cleaned up WORKSPACE and changed the test to read carbon sources from testdata

* updated comment

* adapted to new ErrorOr return value

* proto buffer 3.19.2 -> 3.19.4

* changed comment

* Apply suggestions from code review

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

* Apply suggestions from code review

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

* Update executable_semantics/fuzzing/BUILD

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

* addressed review comments

* updated Unimplemented error message

* Addressed review comments

* more review comments

* switched to loading protobuf via rules_proto()

* Ignore protobuf headers in fix_cc_deps.py until the script supports alias rules

* renamed repeated proto fields to be plural

* added @zlib to check_non_test_cc_deps

* Update common/fuzzing/BUILD

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

* review comments

* set is_omitted_expression for return value

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

executable_semantics/interpreter/resolve_names.cpp

@@ -161,7 +161,8 @@ static auto ResolveNames(Expression& expression,
     case ExpressionKind::TypeTypeLiteral:
       break;
     case ExpressionKind::UnimplementedExpression:
-      FATAL() << "Unimplemented";
+      return FATAL_COMPILATION_ERROR(expression.source_loc())
+             << "Unimplemented";
   }
   return Success();
 }