Rollforward of e06c0ac: Blank out value of crosstool_top when C++ toolchain resolution is enabled

Without this change, analysis fails in case --crosstool_top is set to a cc_toolchain_suite that doesn't have a toolchain matching the --cpu value.

This prevented removing --crosstool_top flag, because the default value only supports a limited set of cpus.

PiperOrigin-RevId: 532073937
Change-Id: Ic869540008e38d2a28e7919fb17db3f9791522f6

Author: comius

src/main/java/com/google/devtools/build/lib/rules/cpp/CppConfiguration.java

@@ -300,8 +300,16 @@ public CppConfiguration(BuildOptions options) throws InvalidConfigurationExcepti
       doc = "The label of the target describing the C++ toolchain",
       defaultLabel = "//tools/cpp:toolchain",
       defaultInToolRepository = true)
+  @Nullable
   public Label getRuleProvidingCcToolchainProvider() {
-    return cppOptions.crosstoolTop;
+    if (cppOptions.enableCcToolchainResolution) {
+      // In case C++ toolchain resolution is enabled, crosstool_top flags are not used.
+      // Returning null prevents additional work on the flags values and makes it possible to
+      // remove `--crosstool_top` flags.
+      return null;
+    } else {
+      return cppOptions.crosstoolTop;
+    }
   }
 
   /** Returns the configured current compilation mode. */