bazel protobuf: add AttributeDefinition.cfg_is_host

It records whether an attribute entails a transition to the "host" configuration.

Ideally we would record more information about the transition, but the
interface seems very abstract and the implementations are varied and
complex (arbitrary function logic). The "host" flag is at least part
of the Java interface, part of the Bazel manual, and part of the
Blaze query interface.

PiperOrigin-RevId: 239320132

src/main/java/com/google/devtools/build/lib/runtime/commands/InfoItem.java

@@ -650,6 +650,7 @@ private static byte[] getBuildLanguageDefinition(RuleClassProvider provider) {
         attrPb.setAllowEmpty(!attr.isNonEmpty());
         attrPb.setAllowSingleFile(attr.isSingleArtifact());
         attrPb.setConfigurable(attr.isConfigurable());
+        attrPb.setCfgIsHost(attr.getConfigurationTransition().isHostTransition());
 
         // Encode default value, if simple.
         Object v = attr.getDefaultValueUnchecked();

src/main/protobuf/build.proto

@@ -506,7 +506,8 @@ message AttributeDefinition {
   optional AttributeValue default = 9; // simple (not computed/late-bound) values only
   optional bool executable = 10;       // type=label
   optional bool configurable = 11;
-  optional bool nodep = 12;            // type=label*
+  optional bool nodep = 12;            // label-valued edge does not establish a dependency
+  optional bool cfg_is_host = 13;      // edge entails a transition to "host" configuration
 }
 
 // An AttributeValue represents the value of an attribute.