The attached and freestanding macro attributes use the same parsing
logic and representation, so generalize the "attached" attribute into
a more general "macro role" attribute.
Describe attached macros with the `@attached` attribute, providing the
macro role and affected names as arguments to the macro. The form of
this macro will remain the same as it gains other kinds of attached
macro roles beyond "accessor".
Remove the "accessors" role from `@declaration`, which will be going
away.
Add support for freestanding declaration macros.
- Parse `@declaration` attribute.
- Type check and expand `MacroExpansionDecl`.
Known issues:
- Generic macros are not yet handled.
- Expansion does not work when the parent decl context is `BraceStmt`. Need to parse freestanding declaration macro expansions in `BraceStmt` as `MacroExpansionDecl`, and add expanded decls to name lookup.
This allows us to evolve e.g. token kinds and how attributes are modelled independently between SwiftSyntax and the compiler. It also makes it easier to e.g. add an attribute because you don’t need to create PRs for two repositories.
