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/lambdas/anonymousFunction.fir.txt

@@ -0,0 +1,13 @@
+FILE fqName:<root> fileName:/anonymousFunction.kt
+  PROPERTY name:anonymous visibility:public modality:FINAL [val]
+    FIELD PROPERTY_BACKING_FIELD name:anonymous type:kotlin.Function0<kotlin.Unit> visibility:public [final,static]
+      EXPRESSION_BODY
+        FUN_EXPR type=kotlin.Function0<kotlin.Unit> origin=LAMBDA
+          FUN name:<no name provided> visibility:local modality:FINAL <> () returnType:kotlin.Unit
+            BLOCK_BODY
+              CALL 'public final fun println (): kotlin.Unit [inline] declared in kotlin.io' type=kotlin.Unit origin=null
+    FUN DEFAULT_PROPERTY_ACCESSOR name:<get-anonymous> visibility:public modality:FINAL <> () returnType:kotlin.Function0<kotlin.Unit>
+      correspondingProperty: PROPERTY name:anonymous visibility:public modality:FINAL [val]
+      BLOCK_BODY
+        RETURN type=kotlin.Nothing from='public final fun <get-anonymous> (): kotlin.Function0<kotlin.Unit> declared in <root>'
+          GET_FIELD 'FIELD PROPERTY_BACKING_FIELD name:anonymous type:kotlin.Function0<kotlin.Unit> visibility:public [final,static] ' type=kotlin.Function0<kotlin.Unit> origin=null