build: correct the OS directory on install

Convert the OS name to lower case as is the convention.  This installs
the content to the right location to permit use from an installed
package.

Author: compnerd

CMakeLists.txt

@@ -131,11 +131,13 @@ add_custom_target(check-xctest
                     XCTest
                   USES_TERMINAL)
 
+string(TOLOWER ${CMAKE_SYSTEM_NAME} SWIFT_OS)
+
 install(FILES
           ${CMAKE_CURRENT_BINARY_DIR}/swift/XCTest.swiftdoc
           ${CMAKE_CURRENT_BINARY_DIR}/swift/XCTest.swiftmodule
         DESTINATION
-          ${CMAKE_INSTALL_FULL_LIBDIR}/swift/${CMAKE_SYSTEM_NAME}/${CMAKE_SYSTEM_PROCESSOR})
+          ${CMAKE_INSTALL_FULL_LIBDIR}/swift/${SWIFT_OS}/${CMAKE_SYSTEM_PROCESSOR})
 install(FILES
           ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_SHARED_LIBRARY_PREFIX}XCTest${CMAKE_SHARED_LIBRARY_SUFFIX}
         DESTINATION