Support @_silgen_name C imports for SE-0324

[glessard]

In standard library modules, many C functions are imported using the following pattern:

@_silgen_name("_swift_stdlib_getUnsafeArgvArgc")
internal func _swift_stdlib_getUnsafeArgvArgc(_: UnsafeMutablePointer<Int32>)
  -> UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>

not followed by a function body. This is special-cased by the compiler.

Functions imported this way do not benefit from the SE-0324 relaxed pointer semantics. They should have the same affordance as other C functions. This pattern also appears with the @objc attribute, but much more rarely than with @_silgen_name.

Tracked as rdar://114794258

[glessard]

Functions imported this way should use the newer @extern(c) syntax.