Populate `CMAKE_Swift_IMPLICIT_INCLUDE_DIRECTORIES` with the macOS SDK's
include directory so that we filter such implicit directories out of
Swift targets.
Fixes: #19845
Allow the `Swift` language to be enabled with the Xcode generator for
Xcode >= 6.1. Reject it on other generators and with older Xcode
versions. Since Apple is the only vendor implementing the language
right now, the compiler id can be just `Apple`.