Evan Wilde authored Nov 05, 2023 and Brad King committed Nov 17, 2023

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