K2: Do not fix variables that has yet unprocessed constraints in forks

Otherwise, exception from org.jetbrains.kotlin.resolve.calls.inference.components.ConstraintInjector.TypeCheckerStateForConstraintInjector.fixedTypeVariable
might happen during forks resolution

The test data is extracted from intelliJ FP test

^KT-43296 Fixed

compiler/testData/diagnostics/tests/inference/forks/nestedCallVariableFixationSimple.fir.kt

@@ -0,0 +1,17 @@
+// SKIP_TXT
+interface Generic<K, V>
+
+fun <X, Y> Generic<X, Y>.getValue(x: X): Y = TODO()
+
+class MyPair<A, B>(a: A, b: B)
+
+fun <E, F> foo(x: Generic<E, F>, e: E, c: Generic<Int, String>): MyPair<F, F> {
+    if (c === x && e is Int) {
+        bar(MyPair(x.getValue(e), x.getValue(e)))
+        return MyPair(x.getValue(e), x.getValue(e))
+    }
+
+    return MyPair(x.getValue(e), x.getValue(e))
+}
+
+fun bar(p: MyPair<String, String>) {}