Make getNestedTypeVariables() an utility

compiler/testData/diagnostics/tests/inference/nestedCalls/externalTypeParameter.kt

@@ -0,0 +1,6 @@
+class A<T> {
+    fun <S> foo(s: S): S = s
+    fun <U> bar(<!UNUSED_PARAMETER!>s<!>: U): List<T> = null!!
+
+    fun test() = foo(bar(""))
+}

compiler/testData/diagnostics/tests/inference/nestedCalls/externalTypeParameter.txt

@@ -0,0 +1,11 @@
+package
+
+public final class A</*0*/ T> {
+    public constructor A</*0*/ T>()
+    public final fun </*0*/ U> bar(/*0*/ s: U): kotlin.List<T>
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final fun </*0*/ S> foo(/*0*/ s: S): S
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final fun test(): kotlin.List<T>
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}