cmd/vet: flag unkeyed struct literals when a struct has two fields of the same type

[josharian]

Suggested in #9409.

Note that vet already flags all unkeyed composite struct literals when the type comes from a different package. This would extend checks to same-package types, but with more stringent criteria.

@bradfitz @alandonovan @shurcooL

[bradfitz]

From @alandonovan in the other thread:

Perhaps we should also consider the case when field value in a literal is *assignable* to more than one field. This is a much larger set. For example:

type T struct { uint; int; float64 }
var _ = T{0, 1, 2}