swift234

[Clang importer] Make the "effective context" a core part of ImportedName.

Previously, the "effective context" parameter to importFullName was
used only during the construction of Swift name lookup tables, so we
can associate each declaration with a context. Expand the role of
"effective context" so it is always a part of ImportedName and is also
used by importDecl when actually importing the enum declaration.

This is partially a cleanup, and partially staging for SE-0033, which
will require this functionality more broadly.