Support definitely-not-null generic T!! types in Parsing and FE1.0

^KT-26245 In Progress

compiler/testData/diagnostics/tests/explicitDefinitelyNotNull/overridesJavaAnnotated.fir.kt

@@ -0,0 +1,42 @@
+// !LANGUAGE: +DefinitelyNotNullTypeParameters +ProhibitUsingNullableTypeParameterAgainstNotNullAnnotated
+
+// FILE: A.java
+import org.jetbrains.annotations.*;
+
+public interface A<T> {
+    public T foo(T x) { return x; }
+    @NotNull
+    public T bar(@NotNull T x) {}
+}
+
+// FILE: main.kt
+
+interface B<T1> : A<T1> {
+    override fun foo(x: T1): T1
+    override fun bar(x: T1!!): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>T1!!<!>
+}
+
+interface C<T2> : A<T2> {
+    override fun foo(x: T2!!): T2!!
+    override fun bar(x: T2): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>T2<!>
+}
+
+interface D : A<String?> {
+    override fun foo(x: String?): String?
+    override fun bar(x: String): String
+}
+
+interface E : A<String> {
+    override fun foo(x: String): String
+    override fun bar(x: String): String
+}
+
+interface F : A<String?> {
+    override fun foo(x: String): String
+    override fun bar(x: String?): String?
+}
+
+interface G<T3 : Any> : A<T3> {
+    override fun foo(x: T3): T3
+    override fun bar(x: T3): T3
+}