Consider the return type in overload resolution

I propose we add a mechanism (attribute or language construct) to consider the return type in overload resolution.

The existing way of approaching this problem in F# is to add a dummy parameter with the type of the return type and call the function with its default value.

## Pros and Cons

- More control over overload resolution
- Less cases leading to ambiguity resolution compile-time errors
- Better type inference, since type inference is better when the return type is taken into account.

The disadvantages of making this adjustment to F# are:

- Adding an extra modifier attribute or a language construct.
- It's work.

## Extra information

Estimated cost: S-M

Related suggestions: #819 #821

## 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