swift234

Clang importer: (de-)serialize Swift name lookup tables via module file extensions.

The recently-introduced module file extensions functionality in Clang
allows us to piggy-back the Swift name lookup tables in Clang's module
files. When the Swift name lookup tables are enabled, introduce such a
module file extension, and wire it into the SwiftLookupTable.

The actual contents of the extension block are simple: a single
on-disk hash table mapping base names to "full entries", which store
the context (e.g., a class, protocol, tag, or TU) and the set of
declarations.

Allow lazy resolution of declaration IDs deserialized from the
extension block to Clang declarations, to try to minimize the set of
declarations we must deserialize. Name lookup itself is only used to
dump the Swift name lookup tables and ensure we're round-tripping
properly.