[FIR] Enable ForbidInferringTypeVariablesIntoEmptyIntersection in 2.0

#KT-56377

compiler/testData/diagnostics/testsWithStdLib/greater.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+// !LANGUAGE: -ForbidInferringTypeVariablesIntoEmptyIntersection
 class Expression<T>(val x: T)
 
 class GreaterOp(val expr1: Expression<*>, val expr2: Expression<*>)

compiler/testData/diagnostics/testsWithStdLib/inference/annotationsForResolve/exactAnnotation.fir.kt

@@ -7,5 +7,5 @@ fun test1(l: List<Number>) {
 
     val i: Int = l.firstTyped()
 
-    val s: String = l.<!INFERRED_TYPE_VARIABLE_INTO_EMPTY_INTERSECTION_WARNING!>firstTyped<!>()
+    val s: String = l.<!INFERRED_TYPE_VARIABLE_INTO_EMPTY_INTERSECTION_ERROR!>firstTyped<!>()
 }