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

This reverts commit 272d1b4,
and the follow-up fix in d209d13.

Even after the follow-up fix, building with an empty
CMAKE_INSTALL_PREFIX errors out with errors like this:

    CMake Error at /b/s/w/ir/x/w/llvm-llvm-project/libcxx/modules/CMakeLists.txt:215 (file):
      file RELATIVE_PATH must be passed a full path to the directory:
      lib/x86_64-pc-windows-msvc

Author: mstorsjo

libcxx/modules/CMakeLists.txt

@@ -206,15 +206,9 @@ 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
-  ${ABS_LIBRARY_DIR}
-  ${ABS_MODULES_DIR})
+  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_LIBRARY_DIR}
+  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_MODULES_DIR})
 configure_file(
   "modules.json.in"
   "${LIBCXX_LIBRARY_DIR}/libc++.modules.json"