Introduce FlagGroup.expandIfTrue, expandIfFalse

This cl adds support for expand_if_true and expand_if_false messages
to the flag_group, allowing more elegant design of build variables.
This cl also adds IntegerValue VariableValue subclass.

RELNOTES: NONE.
PiperOrigin-RevId: 140849578
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 4d23893..3e216c0 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
@@ -102,6 +102,10 @@
     optional string iterate_over = 3;
 
     repeated string expand_if_all_available = 4;
+
+    optional string expand_if_true = 5;
+
+    optional string expand_if_false = 6;
   }
 
   // A key/value pair to be added as an environment variable. The value of