SwiftLexicalLookup: Add Syntax Type Helpers for Member-Type Lookup

[filip-sakel]

A follow-up to #3346 for the qualified name-lookup GSoC 2026 project.

Changes

1. Add NominalTypeDeclSyntax (struct, enum, class, actor, or protocol)
   • This type will be used to store the main declaration of a type representation that's undergone structural type resolution
2. Add TypeDeclSyntax (a nominal type declaration, type alias, or associated/generic type)
   • Unqualified type lookup will return TypeDeclSyntax nodes
3. Add TypeLikeSyntax (a TypeSyntax or NominalTypeDeclSyntax)
   • Helps us track where a type-resolution request originated
   • For instance:

     struct A {
        func f(b: B) {} // <- Look up `B` here
     }
     extension A { struct B {} }

     When looking up B, we first look for member types of A, which means we generate a request to fully resolve A. Note that no type syntax references A; instead, we resolve A because the syntax is nested in a nominal-type declaration. TypeLikeSyntax allows us to represent struct A as the syntax initiating the request.
4. Rename old NominalTypeDeclSyntax->NonProtocolNominalTypeDeclSyntax.
   • This type is used internally to implement unqualified lookup

[filip-sakel]

@swift-ci please test

[filip-sakel]

@swift-ci please test