Swift: Add abstraction for compilation mode

Add a `CMAKE_Swift_COMPILATION_MODE` variable and corresponding
`Swift_COMPILATION_MODE` target property to control the compilation
mode.  Select among `wholemodule`, `singlefile`, and `incremental`.

Add policy CMP0157 to remove the default `-wmo` flags in favor of the
abstract setting.

Issue: #25366

Help/variable/CMAKE_Swift_COMPILATION_MODE.rst

@@ -0,0 +1,32 @@
+CMAKE_Swift_COMPILATION_MODE
+----------------------------
+
+.. versionadded:: 3.29
+
+Specify how Swift compiles a target. This variable is used to initialize the
+:prop_tgt:`Swift_COMPILATION_MODE` property on targets as they are created.
+
+The allowed values are:
+
+.. include:: ../prop_tgt/Swift_COMPILATION_MODE-VALUES.txt
+
+Use :manual:`generator expressions <cmake-generator-expressions(7)>` to support
+per-configuration specification. For example, the code:
+
+.. code-block:: cmake
+
+   set(CMAKE_Swift_COMPILATION_MODE
+     "$<IF:$<CONFIG:Release>,wholemodule,incremental>")
+
+sets the default Swift compilation mode to wholemodule mode when building a
+release configuration and to incremental mode in other configurations.
+
+If this variable is not set then the :prop_tgt:`Swift_COMPILATION_MODE` target
+property will not be set automatically. If that property is unset then CMake
+uses the default value ``incremental`` to build the Swift source files.
+
+.. note::
+
+   This property only has effect when policy :policy:`CMP0157` is set to ``NEW``
+   prior to the first :command:`project` or :command:`enable_language` command
+   that enables the Swift language.