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()

Tests/RunCMake/CMakeLists.txt

@@ -752,7 +752,8 @@ add_RunCMake_test(CheckCompilerFlag   -DCMake_TEST_CUDA=${CMake_TEST_CUDA}
 add_RunCMake_test(CheckSourceCompiles -DCMake_TEST_CUDA=${CMake_TEST_CUDA}
                                       -DCMake_TEST_ISPC=${CMake_TEST_ISPC}
                                       -DCMAKE_Fortran_COMPILER_ID=${CMAKE_Fortran_COMPILER_ID}
-                                      -DCMake_TEST_HIP=${CMake_TEST_HIP})
+                                      -DCMake_TEST_HIP=${CMake_TEST_HIP}
+                                      -DCMake_TEST_Swift=${CMake_TEST_Swift})
 add_RunCMake_test(CheckSourceRuns     -DCMake_TEST_CUDA=${CMake_TEST_CUDA}
                                       -DCMAKE_Fortran_COMPILER_ID=${CMAKE_Fortran_COMPILER_ID}
                                       -DCMake_TEST_HIP=${CMake_TEST_HIP})

Tests/RunCMake/CheckSourceCompiles/CheckSourceCompilesSwift.cmake

@@ -0,0 +1,15 @@
+enable_language(Swift)
+include(CheckSourceCompiles)
+
+set(Swift 1) # test that this is tolerated
+
+check_source_compiles(Swift "baz()" SHOULD_FAIL)
+
+if(SHOULD_FAIL)
+  message(SEND_ERROR "invalid Swift source didn't fail.")
+endif()
+
+check_source_compiles(Swift "print(\"Hello, CMake\")" SHOULD_BUILD)
+if(NOT SHOULD_BUILD)
+  message(SEND_ERROR "Test failed for valid Swift source.")
+endif()

Tests/RunCMake/CheckSourceCompiles/RunCMakeTest.cmake

@@ -31,3 +31,7 @@ endif()
 if(CMake_TEST_HIP)
   run_cmake(CheckSourceCompilesHIP)
 endif()
+
+if(CMake_TEST_Swift)
+  run_cmake(CheckSourceCompilesSwift)
+endif()