Use negative UninterpretedOption field tags for interpreted option locations.

This allows us to extend improved SourceCodeInfo support to dot-notation options without breaking backwards compatibility. Dot-notation options like `(my_opt).a = 123` already emit SourceCodeInfo location in a form of `[..., 10101, 1]` where 1 is the tag of field `a`. That location spans the whole option. But these pre-existing locations are limited as they don't provide granular information about each part of the option (e.g. location of `a`) and don't provide location for value too.

Using negative values allows us to create non-overlapping set of locations. For example in the example above we will emit (in future CL):

[..., 10101] - location of the whole `(my_opt).a = 123` option
[..., 10101, 1] - legacy duplicate location of the whole `(my_opt).a = 123` option
[..., 10101, -2] - location of `(my_opt)` part
[..., 10101, -8, 1, -2] - location of `a` part
[..., 10101, -8, 1, -4] - location of `123` part

PiperOrigin-RevId: 958626527

src/google/protobuf/descriptor.proto

@@ -1406,32 +1406,39 @@ message SourceCodeInfo {
     //       .options()           // 7
     //       .my_opt()            // 10101
     //
-    // Sub-locations corresponding to the interpreted option's corresponding
-    // `UninterpretedOption` are also appended to the interpreted option, which
-    // deviates from the actual FileDescriptorProto path. E.g.:
-    //   [ 4, 3, 7, 10101, 2 ]
-    // refers to the option name `(my_opt)`, and:
-    //   [ 4, 3, 7, 10101, 7 ]
-    // refers to the "foo" string value of the option.
+    // Option parts, e.g. name and value are also appended using field numbers
+    // from `UninterpretedOption`, which deviates from the actual
+    // FileDescriptorProto path and uses negative values.
+    // E.g., for `(my_opt) = "foo"` the name `(my_opt)` is:
+    //   [ 4, 3, 7, 10101, -2 ]
+    // where -2 is negative of `UninterpretedOption.name`.
+    // The value "foo" is:
+    //   [ 4, 3, 7, 10101, -7 ]
+    // where -7 is the negative of `UninterpretedOption.string_value`.
     //
     // For complex options (e.g., "(my_opt) = {a: 100}"), the path
-    // will include UninterpretedOption.aggregate_value (field number 8) as a
+    // will include -UninterpretedOption.aggregate_value (field number -8) as a
     // marker for each level of nesting.
     //
     // For example, given:
     //   option (my_opt) = {a: 100};
     //
     // The path for the `a` identifier would look like:
-    //   [ 4, 3, 7, 10101, 8, 1, 2 ]
+    //   [ 4, 3, 7, 10101, -8, 1, -2 ]
     //
     // And for the value 100:
-    //   [ 4, 3, 7, 10101, 8, 1, 4 ]
+    //   [ 4, 3, 7, 10101, -8, 1, -4 ]
     //
     // Where:
-    //   8: UninterpretedOption.aggregate_value marker
-    //   1: The field number of "a" inside "my_opt"
-    //   2: UninterpretedOption.name
-    //   4: UninterpretedOption.positive_int_value
+    //   -8: UninterpretedOption.aggregate_value marker
+    //    1: The field number of "a" inside "my_opt"
+    //   -2: UninterpretedOption.name
+    //   -4: UninterpretedOption.positive_int_value
+    //
+    // We use negative values for UninterpretedOption due to keep it backward
+    // compatible with pre-existing undocumented behavior where options using
+    // dot-notation (e.g. `(my_opt).a = 100`) also produce a path like
+    // [ 4, 3, 7, 10101, 1] that spans the entire option.
     repeated int32 path = 1 [packed = true];
 
     // Always has exactly three or four elements: start line, start column,

src/google/protobuf/option_interpreter.cc

@@ -1009,7 +1009,7 @@ bool OptionInterpreter::SetAggregateOption(const FieldDescriptor* option_field,
           UninterpretedOption::kAggregateValueFieldNumber);
       SourceCodePath mutable_dest_path = dest_path;
       mutable_dest_path.push_back(
-          UninterpretedOption::kAggregateValueFieldNumber);
+          -UninterpretedOption::kAggregateValueFieldNumber);
       CollectAggregateFieldLocations(*dynamic, info_tree, mutable_src_path,
                                      mutable_dest_path);
     }
@@ -1060,7 +1060,7 @@ void OptionInterpreter::CollectAggregateFieldLocations(
         const TextFormat::ParseInfoTree* sub_tree =
             tree.GetTreeForNested(field, val_marker_idx);
         if (sub_tree != nullptr) {
-          dest_path.push_back(UninterpretedOption::kAggregateValueFieldNumber);
+          dest_path.push_back(-UninterpretedOption::kAggregateValueFieldNumber);
           CollectAggregateFieldLocations(sub_message, *sub_tree,
                                          uninterpreted_path, dest_path);
           dest_path.pop_back();