IrFunctionExpression: add 'origin'

This is actually either a LAMBDA or an ANONYMOUS_FUNCTION.
Not quite sure if it's really required, but some tools such as IR-based
decompiler might require this information.

compiler/testData/ir/irText/expressions/enumEntryReferenceFromEnumEntryClass.fir.txt

@@ -43,7 +43,7 @@ FILE fqName:<root> fileName:/enumEntryReferenceFromEnumEntryClass.kt
       PROPERTY name:aLambda visibility:public modality:FINAL [val] 
         FIELD PROPERTY_BACKING_FIELD name:aLambda type:kotlin.Function0<kotlin.Unit> visibility:public [final]
           EXPRESSION_BODY
-            FUN_EXPR type=kotlin.Function0<kotlin.Unit>
+            FUN_EXPR type=kotlin.Function0<kotlin.Unit> origin=LAMBDA
               FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> ($this:<root>.MyEnum.Z) returnType:kotlin.Unit
                 $this: VALUE_PARAMETER name:<this> type:<root>.MyEnum.Z 
                 BLOCK_BODY