CMake: Improve missing swift warning

Moving the missing Swift compiler warning from a status message to a
warning to make it more prominent. Also providing additional information
on how to fix the situation if it's unexpected.

Author: etcwilde

CMakeLists.txt

@@ -108,8 +108,9 @@ if(CMAKE_Swift_COMPILER)
   enable_language(Swift)
   set(DEFAULT_SWIFT_MIN_RUNTIME_VERSION "${CMAKE_Swift_COMPILER_VERSION}")
 else()
-  message(STATUS "WARNING! Did not find a host compiler swift?! Can not build
-                  any compiler host sources written in Swift")
+  message(WARNING "Swift compiler not found on path.
+  Cannot build compiler sources written in Swift.
+  If this is unexpected, please pass the path to the swiftc binary by defining the `CMAKE_Swift_COMPILER` variable.")
   set(DEFAULT_SWIFT_MIN_RUNTIME_VERSION)
 endif()