CheckSourceCompiles: Add support for Swift

Plumb through swift `check_source_compiles` support.  Add tests to check
that valid swift sources compile and invalid sources don't.

Modules/Internal/CheckSourceCompiles.cmake

@@ -34,6 +34,9 @@ function(CMAKE_CHECK_SOURCE_COMPILES _lang _source _var)
     elseif(_lang STREQUAL "OBJCXX")
       set(_lang_textual "Objective-C++")
       set(_lang_ext "mm")
+    elseif(_lang STREQUAL "Swift")
+      set(_lang_textual "Swift")
+      set(_lang_ext "swift")
     else()
       message (SEND_ERROR "check_source_compiles: ${_lang}: unknown language.")
       return()