FIR IDE: add helper function for getting psi of KtSymbol

idea/idea-frontend-api/src/org/jetbrains/kotlin/idea/frontend/api/symbols/KtSymbol.kt

@@ -16,6 +16,22 @@ interface KtSymbol : ValidityTokenOwner {
     fun createPointer(): KtSymbolPointer<KtSymbol>
 }
 
+/**
+ * Get symbol [PsiElement] if its type is [PSI], otherwise throws ClassCastException
+ *
+ * @see KtSymbol.psi
+ */
+inline fun <reified PSI : PsiElement> KtSymbol.psi(): PSI =
+    psi as PSI
+
+/**
+ * Get symbol [PsiElement] if its type is [PSI], otherwise null
+ *
+ * @see KtSymbol.psi
+ */
+inline fun <reified PSI : PsiElement> KtSymbol.psiSafe(): PSI? =
+    psi as? PSI
+
 /**
  * A place where [KtSymbol] came from
  */