FIR/UAST: commonize literal expressions

plugins/uast-kotlin-fir/testData/declaration/facade.log.fir.txt

@@ -1,7 +1,7 @@
 UFile (package = declaration)
     UClass (name = Utils)
         UMethod (name = foo)
-            [!] UnknownKotlinExpression (INTEGER_CONSTANT)
+            ULiteralExpression (value = "not-yet-compile-time-constant")
         UMethod (name = buzz)
             UParameter (name = $this$buzz)
             [!] UnknownKotlinExpression (BLOCK)

plugins/uast-kotlin-fir/testData/declaration/facade.render.fir.txt

@@ -1,6 +1,6 @@
 package declaration
 
 public final class Utils {
-    public static final fun foo() : int = [!] UnknownKotlinExpression (INTEGER_CONSTANT)
+    public static final fun foo() : int = "not-yet-compile-time-constant"
     public static final fun buzz($this$buzz: java.lang.String) : java.lang.String = [!] UnknownKotlinExpression (BLOCK)
 }