Diff against a dedicated exec version of the baseline configuration

When `--experimental_output_directory_naming_scheme` is set to
`diff_against_baseline`, the current configuration is diffed against a
baseline and a hash of the diff is appended to the output directory.

If a target that is built in the exec configuration transitions further,
the diff would, before this change, include the (substantial) set of
flags that differ between the top-level target and exec configuration.
This made caching less efficient across builds that change flags that
are reset by the exec configuration, e.g. `--collect_code_coverage`.

This is fixed by comparing exec configuration against a dedicated exec
version of the top-level target configuration, which is safe since
`is exec configuration` itself is explicit in the output path.

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

@@ -100,7 +100,9 @@ public SkyValue compute(SkyKey skyKey, Environment env)
       if (platformMappingValue == null) {
         return null;
       }
-      BuildOptions baselineOptions = PrecomputedValue.BASELINE_CONFIGURATION.get(env);
+      BuildOptions baselineOptions = targetOptions.get(CoreOptions.class).isExec
+          ? PrecomputedValue.EXEC_BASELINE_CONFIGURATION.get(env)
+          : PrecomputedValue.BASELINE_CONFIGURATION.get(env);
       try {
         BuildOptions mappedBaselineOptions =
             BuildConfigurationKey.withPlatformMapping(platformMappingValue, baselineOptions)

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

@@ -98,6 +98,8 @@ public static <T> Injected injected(Precomputed<T> precomputed, T value) {
   // Unsharable because of complications in deserializing BuildOptions on startup due to caching.
   public static final Precomputed<BuildOptions> BASELINE_CONFIGURATION =
       new Precomputed<>("baseline_configuration", /*shareable=*/ false);
+  public static final Precomputed<BuildOptions> EXEC_BASELINE_CONFIGURATION =
+      new Precomputed<>("exec_baseline_configuration", /*shareable=*/ false);
 
   private final Object value;
 

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

@@ -1223,6 +1223,8 @@ private void setStarlarkSemantics(StarlarkSemantics starlarkSemantics) {
 
   public void setBaselineConfiguration(BuildOptions buildOptions) {
     PrecomputedValue.BASELINE_CONFIGURATION.set(injectable(), buildOptions);
+    PrecomputedValue.EXEC_BASELINE_CONFIGURATION.set(injectable(),
+        buildOptions.createExecOptions());
   }
 
   public void injectExtraPrecomputedValues(List<PrecomputedValue.Injected> extraPrecomputedValues) {