Swift: Avoid module-wide indexing when naming imported extensions #22771

Description

SwiftMangler::visitExtensionDecl obtains a discriminator through getExtensionOrFilePrivateValueIndex, which enumerates the imported module's top-level declarations. For Clang-backed extensions, this materializes unrelated declarations merely to construct an internal identity.

A real-AST synthetic fixture with two imported Objective-C categories allocates 34,736 additional AST bytes during mangling with no unrelated C structs, and 279,568 bytes with 200 unrelated structs. The equivalent global-as-member fixture grows from 23,944 to 268,776 bytes. These figures measure work in the mangler, not whole-build savings.

Steps to Reproduce

1. Define an Objective-C root class and categories in nested Clang submodules, or C globals mapped into Swift members with swift_name.
2. Add otherwise-unused C structs to the module.
3. Typecheck Swift references, then name the referenced imported members with SwiftRecursiveMangler while measuring NumASTBytesAllocated before and after mangling.

Expected Behavior

Naming already-referenced imported extensions should not import unrelated declarations from the enclosing SDK/module.

Actual Behavior

AST allocation during mangling scales with unrelated module contents.

Environment

• Upstream revision: 36994cdec4ffab77993ac386c85cf551e28f552f
• Bundled native frontend: Swift 6.3.3
• Native standalone build on macOS

Additional Context

A proposed focused correction uses Clang's category USR, and the Swift importer's existing nominal-type/Clang-submodule grouping for global-member extensions. This intentionally changes internal imported-extension TRAP keys; Swift-source/fileprivate indexing and the recursive fallback from #22234 must remain intact. Regression coverage includes nested submodules, late imports, reference reordering and identity uniqueness. No compiler filters, caching workaround or downstream wrapper is proposed.

AI assistance: GitHub Copilot assisted with implementation and validation.