OBJCXX: Fix regression for compiling cpp files as objcxx

In commit 8d61294c (PCH: Mark CMake PCH source files as -x
<lang>-header, 2020-09-04, v3.18.3~14^2) we removed the explicit `-x
objective-c++` flag.  This broke cases with custom source extensions.

Restore the explicit `-x objective-c[++]` flag and put it before the
`<FLAGS>` placeholder.  The latter will contain the proper `-x
objective-c[++]-header` value and will override the `-x objective-c[++]`
value set before.

Fixes: #21234

Modules/CMakeOBJCInformation.cmake

@@ -170,7 +170,7 @@ endif()
 # compile an Objective-C file into an object file
 if(NOT CMAKE_OBJC_COMPILE_OBJECT)
   set(CMAKE_OBJC_COMPILE_OBJECT
-    "<CMAKE_OBJC_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -x objective-c -o <OBJECT> -c <SOURCE>")
+    "<CMAKE_OBJC_COMPILER> <DEFINES> <INCLUDES> -x objective-c <FLAGS> -o <OBJECT> -c <SOURCE>")
 endif()
 
 if(NOT CMAKE_OBJC_LINK_EXECUTABLE)

Modules/CMakeOBJCXXInformation.cmake

@@ -263,7 +263,7 @@ endif()
 # compile an Objective-C++ file into an object file
 if(NOT CMAKE_OBJCXX_COMPILE_OBJECT)
   set(CMAKE_OBJCXX_COMPILE_OBJECT
-    "<CMAKE_OBJCXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> -c <SOURCE>")
+    "<CMAKE_OBJCXX_COMPILER> <DEFINES> <INCLUDES> -x objective-c++ <FLAGS> -o <OBJECT> -c <SOURCE>")
 endif()
 
 if(NOT CMAKE_OBJCXX_LINK_EXECUTABLE)

Tests/ObjCXX/CMakeLists.txt

@@ -2,3 +2,4 @@ ADD_TEST_MACRO(ObjCXX.ObjC++ ObjC++)
 ADD_TEST_MACRO(ObjCXX.simple-build-test simple-build-test)
 ADD_TEST_MACRO(ObjCXX.cxx-file-extension-test cxx-file-extension-test)
 ADD_TEST_MACRO(ObjCXX.objcxx-file-extension-test objcxx-file-extension-test)
+ADD_TEST_MACRO(ObjCXX.cxx-as-objcxx cxx-as-objcxx)

Tests/ObjCXX/cxx-as-objcxx/CMakeLists.txt

+cmake_minimum_required(VERSION 3.18)
+project(cxx-as-objcxx LANGUAGES OBJCXX)
+
+add_executable(cxx-as-objcxx main.cpp)
+set_source_files_properties(main.cpp PROPERTIES LANGUAGE OBJCXX)

Tests/ObjCXX/cxx-as-objcxx/main.cpp

+#import <Foundation/Foundation.h>
+
+int main(int argc, char* argv[])
+{
+  return 0;
+}