Avoid creating \dev\null file when building on Windows (#1697)

* Avoid creating \dev\null file on Windows

* Use a real file path

* Use -typecheck instead of a dummy output file

* Remove input file to -typecheck swiftc call

* Fix invocation

Author: tristanlabelle

CMakeLists.txt

@@ -37,9 +37,7 @@ execute_process(
     "${CMAKE_Swift_COMPILER}"
     -Xfrontend -disable-implicit-string-processing-module-import
     -Xfrontend -parse-stdlib
-    -c - -o /dev/null
-  INPUT_FILE
-    "${CMAKE_BINARY_DIR}/tmp/empty-check-string-processing.swift"
+    -typecheck "${CMAKE_BINARY_DIR}/tmp/empty-check-string-processing.swift"
   OUTPUT_QUIET ERROR_QUIET
   RESULT_VARIABLE
     SWIFT_SUPPORTS_DISABLE_IMPLICIT_STRING_PROCESSING_MODULE_IMPORT)