[SE-0393] Require the repeat keyword for generic requirement expansions.

[hborla]

Implementation for the first revision in swiftlang/swift-evolution#2004

Require the repeat keyword for generic requirements that capture type parameter packs, which are called generic requirement expansions, as they expand out to zero or more generic requirements under pack substitution.

func requirement<each T>() where repeat G<each T>.A == Int {}

Under substitution T := Pack{Int, String, Bool} the requirement expansion repeat G<each T>.A == Int expands into the following list of requirements: G<Int>.A == Int, G<String>.A == Int, G<Bool>.A == Int.

[hborla]

@swift-ci please smoke test