swift234

Clang importer: introduce "importFullName" to centralize name-mapping logic.

The Swift lookup tables are the primary client and test vehicle right
now. This change adds the capability to use the swift_name attribute
to rename C functions when they are imported into Swift, as well as
handling the swift_private attribute more uniformly.

There are a few obvious places where I've applied this API to
eliminate redundancy. Expect it to broaden as the API fills out more.