Corresponding Swift Syntax PR: swiftlang/swift-syntax#3298

According to SE-0415, body macros should be supported on variables and accessors:

Function body macros can be applied to accessors as well, in which case they go on the accessor itself, e.g.,

var area: Double {
  @Logged get {
    return length * width
  }
}

When using the shorthand syntax for get-only properties, a function body macro can be applied to the property itself:

@Logged var area: Double {
  return length * width
}

However, both forms are currently unsupported with errors like error: 'body' macro cannot be attached to var ('area') or error: Declaration is not a type with an optional code block.

This PR adds implements supports for both of these use cases.

• To support body macros on accessors, we only have to fix an issue where the SwiftSyntax plugin did not properly parse the AccessorDeclSyntax received via a plugin message.
• To support body macros on computed property declarations, we have to update the compiler to pass through the VariableDeclSyntax, and then within Swift Syntax synthesize an implicit AccessorDeclSyntax to use in the expansion(of:providingBodyFor:in:) call (which needs a WithOptionalCodeBlockSyntax).

At Airbnb, we'd like to use this functionality to enable adding instrumentation to SwiftUI view bodies, like:

struct MyView: View {
  @Instrumented // Enabled by this fix :)
  var body: some View {
    Text("Hello world!")
  }
}

Fixes #75715.

Please review: @DougGregor @ahoppen