Go: Improve two class names and add some helper predicates

[owen-mc]

This pull request introduces deprecations and enhancements to the Go QL library, focusing on improving the link between declarations and the things that have been declared. Key changes include the deprecation of BuiltinType and DeclaredType in favor of new replacements, the addition of new methods for going from type and field declarations to the type and fields that have been declared, and updates to tests.

[Copilot]

Pull Request Overview

This PR renames two core QL classes to better reflect their roles, deprecates the old names, and adds helper methods and predicates to navigate between declarations and the entities they declare.

• Rename DeclaredType → DeclaredTypeEntity and BuiltinType → BuiltinTypeEntity, with deprecated aliases for backward compatibility.
• Introduce new methods on TypeSpec (getDeclaredType, getRhsType) and on FieldDecl (getField, isEmbedded), plus updated docs.
• Update library tests to exercise the new helpers and adjust expected outputs accordingly.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/ql/test/library-tests/semmle/go/Types/FieldDecl.ql	Added test predicates for getField and isEmbedded.
go/ql/test/library-tests/semmle/go/Types/FieldDecl.expected	Updated expected output to include the new field declaration tests.
go/ql/test/library-tests/semmle/go/Decl/TypeSpec.ql	Expanded select to output getDeclaredType and getRhsType.
go/ql/test/library-tests/semmle/go/Decl/TypeSpec.expected	Adjusted expected columns to match the expanded select clause.
go/ql/lib/semmle/go/Scopes.qll	Renamed DeclaredType/BuiltinType classes to *Entity versions and added deprecated aliases; updated builtin type generators.
go/ql/lib/semmle/go/Decls.qll	Added getDeclaredType, getRhsType, getField, isEmbedded methods and updated related documentation.
go/ql/lib/change-notes/2025-06-05-deprecate-DeclaredType-BuiltinType.md	Documented deprecation of the old class names.