Generate const val initializers as IrConst expressions

compiler/testData/ir/irText/declarations/constValInitializers.kt

@@ -0,0 +1,8 @@
+const val I0 = 0
+const val I1 = 1
+const val I2 = I0 + I1 + I1
+
+const val STR1 = "String1"
+const val STR2 = "String" + "2"
+const val STR3 = STR1 + STR2
+const val STR4 = "$STR1$STR2"