swift234

Clean up handling of external declarations, NFC

This is the first in a series of patches that fixes some resilience-related
issues with synthesized accessors and materializeForSet.

Previously we maintained two lists of external declarations encountered while
type checking:

- ASTContext::ExternalDefinitions
- TypeChecker::implicitlyDefinedFunctions

The former contained the following:

- Imported nominal types from Clang, so that SILGen can emit witness tables
- Functions and variables with Clang decls, so that IRGen can instruct Clang
  to emit them
- Synthesized accessors

The latter contained synthesized functions for derived conformances.

Since the second list was not visible outside Sema, we relied on the Clang
importer to add the type that contained the declaration to the
ExternalDefinitions list. In practice, we only synthesized members of enums
in this manner.

Because of this, SILGenModule::emitExternalDefinitions() had special logic to
skip members of enums, since it would visit them when visiting the enum itself.

Instead, it appears that we can remove implicitlyDefinedFunctions completely,
changing usage sites to add the decl to ExternalDefinitions instead, and
simplify SILGenModule::emitExternalDefinition() a bit in the process.

Also, it looks like we never had Modules appear in ExternalDefinitions, so
assert if those come up instead of skipping them.