[FIR] Add checker for uninitialized properties

compiler/testData/diagnostics/tests/when/ExhaustiveValOverConditionalInit.fir.kt

@@ -18,5 +18,5 @@ fun bar(a: Boolean, b: Boolean): Int {
     when (b) {
         false -> x = 3
     }
-    return x
+    return <!UNINITIALIZED_VARIABLE!>x<!>
 }

compiler/testData/diagnostics/tests/when/ExhaustiveVarOverConditionalInit.fir.kt

@@ -18,5 +18,5 @@ fun bar(a: Boolean, b: Boolean): Int {
     when (b) {
         false -> x = 3
     }
-    return x
+    return <!UNINITIALIZED_VARIABLE!>x<!>
 }

compiler/testData/diagnostics/tests/when/withSubjectVariable/unsupportedVariableDeclarationsInWhenSubject.fir.kt

@@ -12,7 +12,7 @@ fun testSimpleValInWhenSubject() {
 
 fun testValWithoutInitializerWhenSubject() {
     when (val y: Any) {
-        is String -> y.<!UNRESOLVED_REFERENCE!>length<!>
+        is String -> <!UNINITIALIZED_VARIABLE!>y<!>.<!UNRESOLVED_REFERENCE!>length<!>
     }
 }