Update CMakeLists.txt

Remove flag that is unsupported on 5.9

Author: compnerd

stdlib/public/Cxx/CMakeLists.txt

@@ -6,7 +6,7 @@ if(SWIFT_STDLIB_SUPPORT_BACK_DEPLOYMENT)
   list(APPEND SWIFT_CXX_DEPS copy-legacy-layouts)
 endif()
 
-add_swift_target_library(swiftCxx STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY IS_FRAGILE
+add_swift_target_library(swiftCxx STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY
     CxxConvertibleToCollection.swift
     CxxDictionary.swift
     CxxPair.swift