Xcode has a separate setting for Swift compile definitions, so we can
compute a dedicated value for it.  Therefore we can:

* Support the COMPILE_LANGUAGE generator expression for Swift-specific
  filters.

* Avoid passing the `=value` part of definitions, which Swift does
  not support.

This revises commit 5cb625eb (Xcode: Pass compile definitions to
Swift, 2022-06-19, v3.25.0-rc1~493^2) and reverts commit 12c6fec6
(Xcode: Drop CMAKE_INTDIR= definition in Swift targets, 2022-09-30,
v3.25.0-rc1~60^2~2), as the latter is no longer needed.

Fixes: #24086