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/samAdapter.txt

@@ -3,7 +3,7 @@ FILE fqName:<root> fileName:/samAdapter.kt
     BLOCK_BODY
       VAR name:hello type:java.lang.Runnable [val] 
         TYPE_OP type=java.lang.Runnable origin=SAM_CONVERSION typeOperand=java.lang.Runnable
-          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 <> () returnType:kotlin.Unit 
               BLOCK_BODY
                 RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.Unit declared in <root>.test1'