Bump Swift version to 4.0

Author: tkremenek

CMakeLists.txt

@@ -105,7 +105,7 @@ set(SWIFT_ANALYZE_CODE_COVERAGE FALSE CACHE STRING
 set_property(CACHE SWIFT_ANALYZE_CODE_COVERAGE PROPERTY
     STRINGS FALSE "NOT-MERGED" "MERGED")
 
-set(SWIFT_VERSION "3.1" CACHE STRING
+set(SWIFT_VERSION "4.0" CACHE STRING
     "The user-visible version of the Swift compiler")
 set(SWIFT_VENDOR "" CACHE STRING
     "The vendor name of the Swift compiler")