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.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // WITH_RUNTIME
 
 val anonymous = fun() { println() }