Support only simple declarations for 'impl' type aliases

E.g. 'impl typealias Foo<A, B> = Bar<A, B>' is allowed; everything else
(variance, changing order of parameters, etc.) is pretty much disallowed.

This is done for simplicity: otherwise matching the platform/impl class scopes
would be not so straightforward

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -12576,6 +12576,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     doTest(fileName);
                 }
 
+                @TestMetadata("genericClassImplTypeAlias.kt")
+                public void testGenericClassImplTypeAlias() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/platformClass/genericClassImplTypeAlias.kt");
+                    doTest(fileName);
+                }
+
                 @TestMetadata("platformClassMember.kt")
                 public void testPlatformClassMember() throws Exception {
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/platformClass/platformClassMember.kt");