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/nestedCallVariableFixation.kt

@@ -0,0 +1,18 @@
+// SKIP_TXT
+// FIR_IDENTICAL
+// FULL_JDK
+// WITH_STDLIB
+// FILE: PyTokenTypes.java
+public class PyTokenTypes {
+    public static final PyTokenTypes LT = new PyTokenTypes();
+}
+
+// FILE: main.kt
+private val comparisonStrings = hashMapOf(
+    PyTokenTypes.LT to "<",
+)
+
+fun findComparisonNegationOperators(x: PyTokenTypes?): Pair<String, String>? {
+    return comparisonStrings.getValue(x) to
+            comparisonStrings.getValue(x)
+}