Allow setting supports_fission crosstool capability using feature

`supports_fission` can now be expressed using 'per_object_debug_info' feature (should be enabled for it to take effect).

This cl is a step towards https://github.com/bazelbuild/bazel/issues/5883. Also
see the rollout doc here:
https://docs.google.com/document/d/1uv4c1zag6KvdI31qdx8C6jiTognXPQrxgsUpVefm9fM/edit#.

Flag removing legacy behavior is https://github.com/bazelbuild/bazel/issues/6861

RELNOTES: None.
PiperOrigin-RevId: 226950450
diff --git a/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto b/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto
index 340dde5..b117ad5 100644
--- a/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto
+++ b/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto
@@ -368,6 +368,7 @@
   optional bool supports_incremental_linker = 41 [default = false];
   // Legacy field, ignored by Bazel.
   optional bool supports_normalizing_ar = 26 [default = false];
+  // Legacy field, use 'per_object_debug_info' feature instead.
   optional bool supports_fission = 43 [default = false];
   // Legacy field, ignored by Bazel.
   optional bool supports_dsym = 51 [default = false];