Swift syntax APIs lack an abstract way of accessing children. The client has to
down-cast a syntax node to the leaf type to access any of its children. However,
some children are common among different syntax kinds, e.g.
DeclAttributeSyntax and DeclMembers. We should allow an abstract way to
access and modify them, so that clients can avoid logic duplication.
This patch adds a mechanism to define new traits and specify satisfied
traits in specific syntax nodes. A trait is a set of common children
and implemented in Swift as a protocol for syntax nodes to conform to.
As a proof-of-concept, we added two traits for now including DeclGroupSyntax
and BracedSyntax.
Resolves: SR-6931 and SR-6916
