Swift: Propagate Swift_MODULE_DIRECTORY as include directory
Teach include directory computation for Swift to implicitly propagate
the Swift_MODULE_DIRECTORY
of all linked targets as include
directories. This is required to ensure that the swiftmodule of a
linked target is accessible to the compiler of the current target.
Fixes: #19272 (closed)
Topic-rename: swift-implicit-module-include-dir
Edited by Brad King