improve AMP join warning

Any module that defines a top-level join gets this error:

Local definition of ‛join’ clashes with a future Prelude name - this will become an error in GHC 7.10, under the Applicative-Monad Proposal.

This file triggers the warning, but it will keep on working when Prelude exports join:

module M1 (M.join) where join x = x

This file will break when Prelude exports join, but there is no warning currently given.

module M2 where
import M1 (join)
f x = join

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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