Make split configuration transitions work with Bazel.

Creating the split configurations in Bazel uncovered an incrementality issue: ConfigurationFactory.hostConfigCache kept state between builds untracked by Skyframe, which is not good, and therefore had to be fixed.

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MOS_MIGRATED_REVID=97106917
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
index b81ea28..b1598f6 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
@@ -209,8 +209,7 @@
   }
 
   /**
-   * Returns the host configuration for this rule; keep in mind that there may be multiple different
-   * host configurations, even during a single build.
+   * Returns the host configuration for this rule. 
    */
   public BuildConfiguration getHostConfiguration() {
     return hostConfiguration;