Xcode: Fix detection of default language standard when given -std= flags

If one uses `CFLAGS='-std=...'` or `CXXFLAGS='-std=...'` then the given
`-std=` flag(s) will always be used.  That effectively changes the
compiler default standard level and extension settings.  Fix the
Xcode generator's compiler id logic to preserve any `-std=` flag
so that the proper defaults are detected.

This problem was exposed by commit 4a0485be7f (cmStandardLevelResolver:
Avoid unnecessary flags, fix unset level logic, 2021-05-29), which
changed the logic to not pass any `-std=` flag if the standard level
and extension settings requested by the project match the default
(`stdIt <= defaultStdIt` became `stdIt < defaultStdIt`).  The new
logic assumes the detected default standard matches what will actually
happen when the project is generated.

Modules/CMakeDetermineCompilerId.cmake

@@ -539,6 +539,7 @@ Id flags: ${testflags} ${CMAKE_${lang}_COMPILER_ID_FLAGS_ALWAYS}
     else()
       set(id_toolset "")
     endif()
+    set(id_lang_version "")
     if("x${lang}" STREQUAL "xSwift")
       if(CMAKE_Swift_LANGUAGE_VERSION)
         set(id_lang_version "SWIFT_VERSION = ${CMAKE_Swift_LANGUAGE_VERSION};")
@@ -549,8 +550,14 @@ Id flags: ${testflags} ${CMAKE_${lang}_COMPILER_ID_FLAGS_ALWAYS}
       else()
         set(id_lang_version "SWIFT_VERSION = 2.3;")
       endif()
-    else()
-      set(id_lang_version "")
+    elseif("x${lang}" STREQUAL "xC" OR "x${lang}" STREQUAL "xOBJC")
+      if(CMAKE_${lang}_COMPILER_ID_FLAGS MATCHES "(^| )(-std=[^ ]+)( |$)")
+        set(id_lang_version "OTHER_CFLAGS = \"${CMAKE_MATCH_2}\";")
+      endif()
+    elseif("x${lang}" STREQUAL "xCXX" OR "x${lang}" STREQUAL "xOBJCXX")
+      if(CMAKE_${lang}_COMPILER_ID_FLAGS MATCHES "(^| )(-std=[^ ]+)( |$)")
+        set(id_lang_version "OTHER_CPLUSPLUSFLAGS = \"${CMAKE_MATCH_2}\";")
+      endif()
     endif()
     if(CMAKE_OSX_DEPLOYMENT_TARGET)
       set(id_deployment_target