DeclGroupSyntax should have introducerKeyword

[ktoso]

Description

It should be possible to .as(DeclGroupSyntax.self)?.introducerKeyword when writing macros in order to easily write diagnostics like "cannot be applied to struct" etc.

Currently one has to do manual if/else of all the kinds we support

Steps to Reproduce

No response

[ahoppen]

Tracked in Apple’s issue tracker as rdar://124247480