The `-wmo` flag added by commit 6063428de7 (Swift: Update default build
flags, 2022-10-03, v3.26.0-rc1~585^2~1) behaves differently with the old
driver. Detect when the old driver is being used, and avoid adding that
flag.
Fixes: #24641
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`.
