Reapply [libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR

This reapplies 272d1b4
(from llvm#85756), which was reverted in
4079370.

In the previous attempt, empty CMAKE_INSTALL_PREFIX was handled
by quoting them, in d209d13.
That made the calls to cmake_path(ABSOLUTE_PATH) succeed, but the
output paths of that weren't actually absolute, which was
required by file(RELATIVE_PATH).

Avoid this issue by appending a slash to CMAKE_INSTALL_PREFIX,
so we always get an absolute path, even if CMAKE_INSTALL_PREFIX
was empty.

Author: mstorsjo

libcxx/modules/CMakeLists.txt

@@ -206,9 +206,18 @@ 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.
+#
+# Using a trailing slash in BASE_DIRECTORY, to produce a seemingly valid
+# absolute path, even if CMAKE_INSTALL_PREFIX is empty.
+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"