Add Xcode.cc, Xcode.clang methods for iOS builds (#16974)

Makes for easier mocking, particularly for testing AOT snapshot build.
diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index 2d255a4..7d1c8ba 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -175,6 +175,14 @@
       return minorVersion >= kXcodeRequiredVersionMinor;
     return false;
   }
+
+  Future<RunResult> cc(List<String> args) {
+    return runCheckedAsync(<String>['xcrun', 'cc']..addAll(args));
+  }
+
+  Future<RunResult> clang(List<String> args) {
+    return runCheckedAsync(<String>['xcrun', 'clang']..addAll(args));
+  }
 }
 
 Future<XcodeBuildResult> buildXcodeProject({