Gregor Jasny authored Apr 01, 2018 and Brad King committed Apr 18, 2018

Apple platforms macOS, iOS, tvOS, and watchOS can all share the same
compiler information.  Rename `Darwin-*` modules to `Apple-*` and load
them all through `CMAKE_EFFECTIVE_SYSTEM_NAME`.  This saves duplication
of 4 * 21 compiler information modules.

Issue: #17870