Conditional let-bound functions

I propose we allow conditional let-bound functions.

The existing way of approaching this problem in F# is having a helper class for the conditional method.
```fs
module M =
type private HelperClass =
[]
static member Log (x:string) = System.Diagnostics.Debug.WriteLine x

// FS0826: The 'ConditionalAttribute' attribute may only be used on members
let [] private log (x:string) =
System.Diagnostics.Debug.WriteLine x
```

## Pros and Cons

The advantages of making this adjustment to F# are
1. Consistency - `let`-bound functions in modules compile to methods. What makes them not allowed to be conditional?
2. Conciseness - A helper class just to define a helper function?

The disadvantages of making this adjustment to F# are none that I can think of.

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

Related suggestions:
https://github.com/dotnet/fsharp/issues/8689 - In contrast, user-defined method-only attributes can be placed on `let`-bound properties and literals??

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this

Start with the proposed example and the related suggestion in issue #8689, then trace the F# compiler's handling of ConditionalAttribute on let-bound functions. Done means a private module-level let-bound function can use ConditionalAttribute without requiring the helper class shown in the issue, while retaining conditional behavior.

Written by the indexing model from the issue text.