Set Swift language version to 5

The stdlib needs to build in Swift-5 mode, and ensure that
`-swift-verion 5` be included in the commandline explicitly. While the
compiler defaults to 5 mode, if it's not included explicitly, it won't
be included in the swift interface, which then means that modules that
import it will influence how the textual interface is processed.
The interface for the stdlib must be processed in the Swift 5 language
mode due to sendability. Set it explicitly to ensure that importing
modules do not change the Swift language mode.

Fixes: rdar://145118843

Author: etcwilde

Runtimes/Core/cmake/modules/CompilerSettings.cmake

@@ -1,7 +1,10 @@
 include(CheckSourceCompiles)
 include(CheckCompilerFlag)
 
-# Use C+17
+# Use Swift 5 mode
+set(CMAKE_Swift_LANGUAGE_VERSION 5)
+
+# Use C++17
 set(SwiftCore_MIN_CXX_STANDARD 17)
 # Unset CMAKE_CXX_STANDARD if it's too low and in the CMakeCache.txt
 if($CACHE{CMAKE_CXX_STANDARD} AND $CACHE{CMAKE_CXX_STANDARD} LESS ${SwiftCore_MIN_CXX_STANDARD})