Removed some special escapes from def_to_proto.c, to match C++ protos.

This was not a bug -- the previous behavior was correct.  However this
change brings our implementation-specific details in line with C++.

PiperOrigin-RevId: 511273853

upb/util/def_to_proto.c

@@ -102,28 +102,20 @@ static bool upb_isprint(char ch) { return ch >= 0x20 && ch <= 0x7f; }
 
 static int special_escape(char ch) {
   switch (ch) {
-    case '\a':
-      return 'a';
-    case '\b':
-      return 'b';
-    case '\f':
-      return 'f';
+    // This is the same set of special escapes recognized by
+    // absl::CEscape().
     case '\n':
       return 'n';
     case '\r':
       return 'r';
     case '\t':
       return 't';
-    case '\v':
-      return 'v';
     case '\\':
       return '\\';
     case '\'':
       return '\'';
     case '"':
       return '"';
-    case '\?':
-      return '?';
     default:
       return -1;
   }

upb/util/def_to_proto_test.cc

@@ -301,4 +301,19 @@ TEST(FuzzTest, PackageStartsWithNumber) {
       ParseTextProtoOrDie(R"pb(file { name: "" package: "0" })pb"));
 }
 
+TEST(FuzzTest, RoundTripDescriptorRegression) {
+  RoundTripDescriptor(ParseTextProtoOrDie(R"pb(file {
+                                                 name: ""
+                                                 message_type {
+                                                   name: "A"
+                                                   field {
+                                                     name: "B"
+                                                     number: 1
+                                                     type: TYPE_BYTES
+                                                     default_value: "\007"
+                                                   }
+                                                 }
+                                               })pb"));
+}
+
 }  // namespace upb_test