FIR: don't check annotations on returnTypeRefs that are not of real kind

This fixes an exception in FirAnnotationChecker where we tried to report
repeated annotations on implicit type refs that have no source.

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerFirTestdataTestGenerated.java

@@ -4870,6 +4870,12 @@ public class DiagnosisCompilerFirTestdataTestGenerated extends AbstractDiagnosis
             runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/removeOnAbstractMap.kt");
         }
 
+        @Test
+        @TestMetadata("repeatedAnnotations.kt")
+        public void testRepeatedAnnotations() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/repeatedAnnotations.kt");
+        }
+
         @Test
         @TestMetadata("runOnIntegerLiteral.kt")
         public void testRunOnIntegerLiteral() throws Exception {