[Swiftify] Add _SwiftifyImportProtocol for safe overloads for protocols

The existing _SwiftifyImport macro is a peer macro, limiting it to only
emitting function wrappers in the same scope as the original function.
Protocols cannot contain function implementations, so these need to be
placed in a separate protocol extension instead. _SwiftifyImportProtocol
is an extension macro rather than a peer macro, to enable this
functionality.

Rather than operating on a single function, like _SwiftifyImport,
_SwiftifyImportProtocol takes information about multiple methods and
creates a single protocol extension with all wrappers in a one-shot
operation.

rdar://144335990

Author: hnrklssn