[libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR (llvm#85756)

In other contexts, install directories such as
LIBCXX_INSTALL_LIBRARY_DIR and LIBCXX_INSTALL_MODULES_DIR can be
specified either as a relative path, relative to CMAKE_INSTALL_PREFIX,
or as an absolute path.

When calculating the relative path between the two, account for the fact
that LIBCXX_INSTALL_MODULES_DIR and LIBCXX_INSTALL_LIBRARY_DIR can be
absolute paths too.

Author: mstorsjo

libcxx/modules/CMakeLists.txt

@@ -206,9 +206,15 @@ add_custom_target(generate-cxx-modules
 # Configure the modules manifest.
 # Use the relative path between the installation and the module in the json
 # file. This allows moving the entire installation to a different location.
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_LIBRARY_DIR
+  BASE_DIRECTORY ${CMAKE_INSTALL_PREFIX}
+  OUTPUT_VARIABLE ABS_LIBRARY_DIR)
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_MODULES_DIR
+  BASE_DIRECTORY ${CMAKE_INSTALL_PREFIX}
+  OUTPUT_VARIABLE ABS_MODULES_DIR)
 file(RELATIVE_PATH LIBCXX_MODULE_RELATIVE_PATH
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_LIBRARY_DIR}
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_MODULES_DIR})
+  ${ABS_LIBRARY_DIR}
+  ${ABS_MODULES_DIR})
 configure_file(
   "modules.json.in"
   "${LIBCXX_LIBRARY_DIR}/libc++.modules.json"