gitlab-ci: test CMAKE_CXX_STDLIB_MODULES_JSON in CI

Do this by relocating the file to a sibling directory so that its `../`
contents continue to find the correct module interface files, but it
won't be found by default.

.gitlab-ci.yml

@@ -478,6 +478,16 @@ t:hip6.3-radeon:
     variables:
         CMAKE_CI_JOB_NIGHTLY: "true"
 
+t:linux-gcc-cxx-modules-reloc-ninja:
+    extends:
+        - .gcc_cxx_modules_reloc_ninja
+        - .cmake_test_linux_release
+        - .linux_x86_64_tags
+        - .run_dependent
+        - .needs_centos7_x86_64
+    variables:
+        CMAKE_CI_JOB_NIGHTLY: "true"
+
 t:linux-gcc-cxx-modules-ninja:
     extends:
         - .gcc_cxx_modules_ninja

.gitlab/ci/configure_linux_gcc_cxx_modules_reloc_ninja.cmake

@@ -0,0 +1,13 @@
+# "Misplace" the `libstdc++.modules.json` file so that
+# `CMAKE_CXX_STDLIB_MODULES_JSON` is needed to use `import std`.
+set(gcc_prefix "/opt/gcc-importstd")
+set(CMake_TEST_CXX_STDLIB_MODULES_JSON
+  "${gcc_prefix}/lib64/libstdc++.modules.json"
+  CACHE FILEPATH "")
+file(MAKE_DIRECTORY
+  "${gcc_prefix}/lib64.reloc")
+file(RENAME
+  "${gcc_prefix}/lib64/libstdc++.modules.json"
+  "${CMake_TEST_CXX_STDLIB_MODULES_JSON}")
+
+include("${CMAKE_CURRENT_LIST_DIR}/configure_external_test.cmake")

.gitlab/os-linux.yml

@@ -524,6 +524,13 @@
     variables:
         CMAKE_CONFIGURATION: linux_gcc_cxx_modules_ninja
 
+.gcc_cxx_modules_reloc_ninja:
+    extends: .gcc_cxx_modules_x86_64
+
+    variables:
+        CMAKE_CONFIGURATION: linux_gcc_cxx_modules_reloc_ninja
+        CTEST_LABELS: "CXXModules"
+
 .gcc_cxx_modules_ninja_multi:
     extends: .gcc_cxx_modules_x86_64