Log a message when we wipe invalidate the transitive closure of all ConfiguredTargets because of a configurations change.

--
MOS_MIGRATED_REVID=96361754
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java b/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java
index 23f9da0..07f5483 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java
@@ -557,6 +557,8 @@
     // data.
     if ((this.configurations != null && !configurations.equals(this.configurations))
         || skyframeAnalysisWasDiscarded) {
+      LOG.info("Discarding analysis cache: configurations have changed.");
+
       skyframeExecutor.dropConfiguredTargets();
       skyframeCacheWasInvalidated = true;
       clear();