c++modules: Persist CMAKE_CXX_STDLIB_MODULES_JSON

And change checks to look for a value rather than definition. This fix
keeps the same set of test cases in `RunCMake.CXXModules` selected as
before.

Fixes: #27589

Modules/CMakeCXXCompiler.cmake.in

@@ -98,6 +98,4 @@ set(CMAKE_CXX_COMPILER_CLANG_RESOURCE_DIR "@CMAKE_CXX_COMPILER_CLANG_RESOURCE_DI
 
 set(CMAKE_CXX_COMPILER_IMPORT_STD "@CMAKE_CXX_COMPILER_IMPORT_STD@")
 set(CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE  "@CMAKE_CXX_COMPILER_IMPORT_STD_ERROR_MESSAGE@")
-if(CMAKE_CXX_STDLIB_MODULES_JSON)
-  set(CMAKE_CXX_STDLIB_MODULES_JSON "@CMAKE_CXX_STDLIB_MODULES_JSON@")
-endif()
+set(CMAKE_CXX_STDLIB_MODULES_JSON "@CMAKE_CXX_STDLIB_MODULES_JSON@")

Modules/Compiler/Clang-CXX-CXXImportStd.cmake

@@ -8,7 +8,7 @@ function (_cmake_cxx_find_modules_json)
     return ()
   endif ()
 
-  if (NOT DEFINED CMAKE_CXX_STDLIB_MODULES_JSON)
+  if (NOT CMAKE_CXX_STDLIB_MODULES_JSON)
     execute_process(
       COMMAND
         "${CMAKE_CXX_COMPILER}"

Modules/Compiler/GNU-CXX-CXXImportStd.cmake

@@ -4,7 +4,7 @@ function (_cmake_cxx_find_modules_json)
     return ()
   endif ()
 
-  if (NOT DEFINED CMAKE_CXX_STDLIB_MODULES_JSON)
+  if (NOT CMAKE_CXX_STDLIB_MODULES_JSON)
     execute_process(
       COMMAND
         "${CMAKE_CXX_COMPILER}"

Modules/Compiler/MSVC-CXX-CXXImportStd.cmake

@@ -1,5 +1,5 @@
 function (_cmake_cxx_find_modules_json)
-  if (DEFINED CMAKE_CXX_STDLIB_MODULES_JSON)
+  if (CMAKE_CXX_STDLIB_MODULES_JSON)
     return ()
   endif ()