`BasicFormat` doesn't add spaces between attributes

[stevapple]

Description

When using AttributeListBuilder, the build result (formatted by BasicFormat) doesn't have spaces between each attributes, which is ill-formed in Swift.

Steps to Reproduce

let builder = AttributeListSyntax {
  AttributeSyntax("@inlinable")
  AttributeSyntax("@discardableResult")
}
print(builder) // @inlinable@discardableResult

[ahoppen]

rdar://107787207

[stevapple]

It also fails to add the space between an attribute and the declaration, eg.

let buildable = ImportDeclSyntax(attributes: [.attribute("@_exported")], path: [.init(name: "MyModule")])
print(buildable) // @_exportedimport MyModule

[ahoppen]

Fixed by #1569