[libc++] Don't commit libcxx.imp (#89391)

We can instead generate it on-the-fly when we install the headers. This
reduces the amount of boilerplate we have to re-generate whenever we
add, remove or relocate header files.

Fixes #88529

Author: ldionne

libcxx/include/CMakeLists.txt

@@ -957,7 +957,6 @@ set(files
   istream
   iterator
   latch
-  libcxx.imp
   limits
   list
   locale
@@ -1036,6 +1035,15 @@ foreach(f ${files})
   list(APPEND _all_includes "${dst}")
 endforeach()
 
+# Generate the IWYU mapping. This depends on all header files but it's also considered as an
+# "include" for dependency tracking.
+add_custom_command(OUTPUT "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp"
+  COMMAND "${Python3_EXECUTABLE}" "${LIBCXX_SOURCE_DIR}/utils/generate_iwyu_mapping.py" "-o" "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp"
+  DEPENDS ${_all_includes}
+  COMMENT "Generate the mapping file for include-what-you-use"
+)
+list(APPEND _all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")
+
 add_custom_target(generate-cxx-headers ALL DEPENDS ${_all_includes})
 
 add_library(cxx-headers INTERFACE)
@@ -1074,6 +1082,12 @@ if (LIBCXX_INSTALL_HEADERS)
     PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     COMPONENT cxx-headers)
 
+  # Install the generated IWYU file to the generic include dir.
+  install(FILES "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp"
+    DESTINATION "${LIBCXX_INSTALL_INCLUDE_DIR}"
+    PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
+    COMPONENT cxx-headers)
+
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_custom_target(install-cxx-headers
                       DEPENDS cxx-headers