Warn about the TailCall attribute on non-rec functions or let-bound values

[dawedawe]

Currently a user can do things like:

[<TailCall>]
let someNonRecFun x = x + x

[<TailCall>]
let someX = 23

[<TailCall>]
let rec someRecLetBoundValue = nameof(someRecLetBoundValue)

While not super harmful, this PR adds a warning for these constructs.

[psfinaki]

Thanks!

[psfinaki]

@dawedawe this is good to go - there is a conflict though (but shouldn't be a big deal).

[dawedawe]

Thanks for the reviews. The conflict is resolved now.