(ImplicitParams) Allow ? in binding patterns

This is mostly useful when dealing with ImplicitParams. Suppose "foo" takes an implicit parameter "?msg"

-- Invalid, parse error in pattern ?msg
bar ?msg = foo
\?msg -> foo

-- With -XViewPatterns, same issue
bar ( ... -> ?msg) = foo

Instead, you are forced to create a local let binding

bar msg = let ?msg = msg in foo
\msg -> let ?msg = msg in foo

bar ( ... -> msg) = let ?msg = msg in foo

Trac metadata

Trac field	Value
Version	8.4.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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