[Tests] Cover @derivative module selector in ModuleSelectorTests

[devdattatalele]

Summary

Adds a substructure assertion inside testModuleSelectorCorrectCode for the modern autodiff registration attribute with a module selector in its of: argument:

@derivative(of: ModuleSelectorTestingKit::negate)
func negateDerivative() {}

The attribute argument list flows through parseQualifiedDeclarationName → parseDeclReferenceExpr, so module selectors are already supported by the parser. The new test locks that behaviour in for the simple decl-reference form, complementing the existing test in testModuleSelectorAttrs (line 1754 in the pre-patch file) which covers the complex member.access form with a wrt: clause.

Context

Addresses the pre-existing FIXME in Tests/SwiftParserTest/translated/ModuleSelectorTests.swift:

// FIXME: Add tests with autodiff @_differentiable(jvp:vjp:) and
// @_derivative(of:)

Both attribute names in the FIXME use their legacy underscore-prefixed spellings. Neither is recognised by the current parser as the special-syntax attribute it refers to:

• @_derivative — the .derivative keyword text is "derivative" (no underscore) in Sources/SwiftSyntax/generated/Keyword.swift. DeclarationAttributeWithSpecialSyntax in Sources/SwiftParser/Attributes.swift matches on this keyword, so source @_derivative is not recognised as a derivative-registration attribute. The modern spelling is @derivative.
• @_differentiable(jvp:vjp:) — the jvp: / vjp: argument labels have been removed from parseDifferentiableAttributeArguments; the modern differentiable attribute uses KindSpecifier, wrt:, and where clauses instead.

The addressable intent of the FIXME — module-selector coverage for a derivative registration attribute inside testModuleSelectorCorrectCode — is satisfied here with the modern @derivative spelling.

The adjacent FIXME about @convention(witness_method:) is a type-attribute concern parsed by different code and is intentionally out of scope.

Test plan

• swift test --test-product SwiftParserTests passes.
• swift format -ipr . produces no diff.
• The new test is inserted inside testModuleSelectorCorrectCode() alongside the existing @_implements and @_dynamicReplacement module-selector coverage. Same substructure pattern as neighbouring tests.
• No changes to parser code or public API.
• Not redundant with the existing @derivative test in testModuleSelectorAttrs (that test covers the complex Swift::Foo.Swift::Bar.Swift::baz() qualified form with wrt: quux; this test covers the simple Module::name decl-reference form).

Files changed

• Tests/SwiftParserTest/translated/ModuleSelectorTests.swift: +30 / -2 lines