add debug message for skipped toolchain resolution

RELNOTES: added additional debug message to warn of skipped toolchains during resolution
PiperOrigin-RevId: 475900663
Change-Id: I6e5c79838c27c9ff416135c3d8afdb79e15c8f9a

Author: Googler

src/main/java/com/google/devtools/build/lib/skyframe/SingleToolchainResolutionFunction.java

@@ -181,6 +181,14 @@ private static SingleToolchainResolutionValue resolveConstraints(
       for (ConfiguredTargetKey executionPlatformKey : availableExecutionPlatformKeys) {
         // Only check the toolchains if this is a new platform.
         if (platformKeysSeen.contains(executionPlatformKey)) {
+          debugMessage(
+              eventHandler,
+              "    Type %s: target platform %s: execution platform %s: Skipping toolchain %s;"
+                  + " execution platform already has selected toolchain",
+              toolchainType.toolchainType(),
+              targetPlatform.label(),
+              executionPlatformKey.getLabel(),
+              toolchain.toolchainLabel());
           continue;
         }