Don't report unused variable warning for delegated variables with provide delegate

^KT-38871 Fixed

compiler/testData/diagnostics/tests/noUnusedOnDelegationWithProvider.kt

@@ -0,0 +1,27 @@
+// FIR_IDENTICAL
+// !DIAGNOSTICS: +UNUSED_VARIABLE
+
+import kotlin.reflect.KProperty
+
+class Example {
+    val valProp: String by Delegate()
+    val varProp: String by Delegate()
+
+    fun foo() {
+        val valVariable by Delegate()
+        val varVariable by Delegate()
+    }
+}
+
+class Delegate {
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): String = "delegation"
+
+    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
+        // setValue
+    }
+
+    operator fun provideDelegate(thisRef: Any?, property: KProperty<*>): Delegate {
+        // side effect
+        return Delegate()
+    }
+}

compiler/testData/diagnostics/tests/unusedVariableOnRegularDelegatedProperty.fir.kt

@@ -0,0 +1,21 @@
+// !DIAGNOSTICS: +UNUSED_VARIABLE
+
+import kotlin.reflect.KProperty
+
+class Example {
+    val valProp: String by Delegate()
+    val varProp: String by Delegate()
+
+    fun foo() {
+        val valVariable by Delegate()
+        val varVariable by Delegate()
+    }
+}
+
+class Delegate {
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): String = "delegation"
+
+    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
+        // setValue
+    }
+}

compiler/testData/diagnostics/tests/unusedVariableOnRegularDelegatedProperty.kt

@@ -0,0 +1,21 @@
+// !DIAGNOSTICS: +UNUSED_VARIABLE
+
+import kotlin.reflect.KProperty
+
+class Example {
+    val valProp: String by Delegate()
+    val varProp: String by Delegate()
+
+    fun foo() {
+        val <!UNUSED_VARIABLE!>valVariable<!> by Delegate()
+        val <!UNUSED_VARIABLE!>varVariable<!> by Delegate()
+    }
+}
+
+class Delegate {
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): String = "delegation"
+
+    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
+        // setValue
+    }
+}