Rename: this object, receiver argument -> dispatch receiver, extension receiver

compiler/testData/resolvedCalls/invoke/bothReceivers.txt

@@ -12,5 +12,5 @@ Resolved call:
 Resulting descriptor: fun Int.invoke(): Unit defined in Foo
 
 Explicit receiver kind = BOTH_RECEIVERS
-This object = f {Foo}
-Receiver argument = 1 {Int}
+Dispatch receiver = f {Foo}
+Extension receiver = 1 {Int}