FirConstChecks: support "Some string".length case properly

#KT-48165 Fixed

compiler/fir/analysis-tests/testData/resolve/constVal/const.fir.txt

@@ -23,6 +23,16 @@ FILE: const.kt
         public get(): R|kotlin/Int|
     public final const val l: R|kotlin/Int| = R|/k|
         public get(): R|kotlin/Int|
+    public final const val m: R|kotlin/String| = String(123).R|kotlin/Any.toString|()
+        public get(): R|kotlin/String|
+    public final const val n: R|kotlin/Int| = String(456).R|kotlin/String.length|
+        public get(): R|kotlin/Int|
+    public final val o: R|kotlin/String| = String(789)
+        public get(): R|kotlin/String|
+    public final const val p: R|kotlin/String| = R|/o|.R|kotlin/Any.toString|()
+        public get(): R|kotlin/String|
+    public final const val q: R|kotlin/Int| = R|/o|.R|kotlin/String.length|
+        public get(): R|kotlin/Int|
     public final class ForConst : R|kotlin/Any| {
         public constructor(): R|ForConst| {
             super<R|kotlin/Any|>()

compiler/fir/analysis-tests/testData/resolve/constVal/const.kt

@@ -18,6 +18,11 @@ const val i = <!CONST_VAL_WITH_NON_CONST_INITIALIZER!>ForConst.one() + "one"<!>
 const val j = <!CONST_VAL_WITH_NON_CONST_INITIALIZER!>4 * ForConst.two()<!>
 val k = 3 - ForConst.two()
 const val l = <!CONST_VAL_WITH_NON_CONST_INITIALIZER!>k<!>
+const val m = "123".toString()
+const val n = "456".length
+val o = "789"
+const val p = <!CONST_VAL_WITH_NON_CONST_INITIALIZER!>o.toString()<!>
+const val q = <!CONST_VAL_WITH_NON_CONST_INITIALIZER!>o.length<!>
 
 class ForConst{
     companion object {

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/FirConstChecks.kt

@@ -160,8 +160,20 @@ internal fun checkConstantArguments(
             }
         }
         expression is FirQualifiedAccessExpression -> {
+            val propertySymbol = expressionSymbol as? FirPropertySymbol ?: return ConstantArgumentKind.NOT_CONST
+
+            @OptIn(SymbolInternals::class)
+            val property = propertySymbol.fir
             when {
-                (expressionSymbol as FirPropertySymbol).isLocal || expressionSymbol.callableId.className?.isRoot == false ->
+                property.name.asString() == "length" -> {
+                    val coneType =
+                        expression.dispatchReceiver.typeRef.coneTypeSafe<ConeKotlinType>() ?: return ConstantArgumentKind.NOT_CONST
+                    val receiverClassId = coneType.lowerBoundIfFlexible().classId
+                    if (receiverClassId == StandardClassIds.String) {
+                        return checkConstantArguments(expression.dispatchReceiver, session)
+                    }
+                }
+                propertySymbol.isLocal || propertySymbol.callableId.className?.isRoot == false ->
                     return ConstantArgumentKind.NOT_CONST
                 expression.typeRef.coneType.classId == StandardClassIds.KClass ->
                     return ConstantArgumentKind.NOT_KCLASS_LITERAL
@@ -170,9 +182,7 @@ internal fun checkConstantArguments(
                 expression.dispatchReceiver is FirThisReceiverExpression ->
                     return null
             }
-            @OptIn(SymbolInternals::class)
+            return when (property.initializer) {
-            val property = expressionSymbol.fir as? FirProperty
-            return when (property?.initializer) {
                 is FirConstExpression<*> -> {
                     if (property.isVal)
                         ConstantArgumentKind.NOT_CONST_VAL_IN_CONST_EXPRESSION