[Macros] Introduce member attribute macros.

[hborla]

For each member of the declaration the macro attribute is attached to, the macro can produce a set of attributes to attach to that member. For example:

@declaration(memberAttribute)
macro wrapStoredProperties: Void

@wrapStoredProperties
struct Point {
  var x: Int
  var y: Int

  var description: String { "" }
}

// Expanded to

struct Point {
  @Wrapper var x: Int
  @Wrapper var y: Int

  var description: String { "" }
}

The member attribute macro expansion is given the declaration it's attached to along with a member declaration, and it returns a set of attributes to attach to the member. The expansion method is called for each member in the attachedTo declaration.

[hborla]

@swift-ci please test

[DougGregor]

This is making me think that there might be a useful abstraction around attached macros (e.g., ones that are written as attributes), and that DeclarationMacro is basically a useless abstraction.

[DougGregor]

Not for your PR, but we should revisit the protocol design here

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test