[SR-4148] Indexed access to a complex type representation

[nkcsgexi]

Previous ID	SR-4148
Radar	None
Original Reporter	@nkcsgexi
Type	New Feature

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	New Feature, Syntax
Assignee	@bitjammer
Priority	Medium

md5: 5332377ec4d12158f94cc129bc84d56d

Issue Description:

For syntax nodes for type repr, we should have a generic way to navigate the complexity of type repr. For instance, from a complex type repr like (C1, (C2, C3?))->C4, we can get the C3 by a given index chain of 0:1:1:0.

API example:

Optional<TypeReprSyntax> getSubTypeRepr(ArrayRef<unsigned> Indices, UnfoundReason &Reason) {  
....  
}

This feature will allow us to bridge syntax tree with swift-api-digester because the later always dumps type change info in a mechanical way, for instance:

TYPE_CHANGE(REMOVE_OPTIONALITY, "objc@someUSRofAFunction", "1"/**second parameter**/, "0:1:1"/**type indices**/) // from api-digester

This tells us the second parameter of the given function should change from
(C1, (C2, C3?))->C4 to
(C1, (C2, C3))->C4.

[AnthonyLatsis]

libSyntax was removed in #62145 and succeeded by swift-syntax.