Allow ViewPatterns in function definitions to refer to where-bound names

Motivation

You can't refer to where-bound variables in ViewPatterns in the LHS of a function definition. It seems like it would be useful to be able to do so. Example:

f (g -> x) = ...
  where
    g = ...

doesn't work, but

f = \(g -> x) -> ...
  where
    g = ...

does

I'm not sure if any issues arise from allowing this.

Proposal

It would be nice if the first example from above also worked.