Since commit 2026915f8f (Swift: Propagate Swift_MODULE_DIRECTORY as include
directory, 2020-02-03, v3.18.0-rc1~547^2) we internally call
`GetAllConfigCompileLanguages` on all directly linked targets without
checking if they are interface libraries that don't compile at all.
That violates an internal assumption and assertion.
Fixes: #20977
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
The check added in commit b06f4c8a74 (Swift: disallow WIN32_EXECUTABLE
properties, 2019-05-31, v3.15.0-rc1~9^2) makes sense only for
executables because the `WIN32_EXECUTABLE` property is defined only for
them. Running the check on other target types, particularly those that
do not link such as INTERFACE libraries, violates internal assumptions.
In particular, `GetLinkerLanguage` should not be called on such targets.
Fixes: #19528
Create a new CMAKE_Swift_LANGUAGE_VERSION variable to specify the
SWIFT_VERSION attribute in a generated Xcode project. Ideally this
would be a `<LANG>_STANDARD` property but since Swift support is
very minimal we should reserve that property for more complete
treatment later.
Issue: #16326
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`.
