[cmake] Add support for checking if we have a host toolchain Swift compiler.

This just causes us to perform the cmake check. It isn't used anywhere.

Author: gottesmm

CMakeLists.txt

@@ -51,6 +51,17 @@ set(CMAKE_CXX_EXTENSIONS NO)
 include(SwiftUtils)
 include(CheckSymbolExists)
 include(CMakeDependentOption)
+include(CheckLanguage)
+
+# Enable Swift for the host compiler build if we have the language. It is
+# optional until we have a bootstrap story.
+check_language(Swift)
+if(CMAKE_Swift_COMPILER)
+  enable_language(Swift)
+else()
+  message(STATUS "WARNING! Did not find a host compiler swift?! Can not build
+                  any compiler host sources written in Swift")
+endif()
 
 #
 # User-configurable options that control the inclusion and default build