The driver infers the filename from the module file by replacing the extension,
and passes the explicit path to the swiftdoc file to the frontend. But there
is no option in the driver to control emission of swiftdoc (it is always
emitted, and name is always inferred from the swiftmodule name).
The swiftdoc file consists of a single table that maps USRs to {brief comment,
raw comment}. In order to look up a comment for decl we generate the USR
first. We hope that the performance hit will not be that bad, because most
declarations come from Clang. The advantage of this design is that the
swiftdoc file is not locked to the swiftmodule file, and can be updated,
replaced, and even localized.
Swift SVN r14914
