[Parser] Add a contextual each keyword for pack references.

[hborla]

• Add PackReferenceTypeRepr for pack references spelled with the each keyword.
• Parse each as a contextual keyword applied to types.

For example:

protocol P {}

struct S<T...> where each T: P {}

For now, each is not required, but the idea is all pack references must be spelled with each, which can only appear inside pack expansions or generic requirements.

[hborla]

swiftlang/swift-syntax#1134

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test