[libc++] Update CMake dependency for generate_iwyu_mapping.py (llvm#115387)

This script does not depend on the generated headers since those are
already special-cased in header_information.py. Change the dependency
list to depend on header_information.py instead. While looking at this
code also simplify the assignment to libcxx_root inside this script.

Author: arichardson

libcxx/include/CMakeLists.txt

@@ -1057,7 +1057,7 @@ endforeach()
 # "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 ${_all_includes}
+  DEPENDS "${LIBCXX_SOURCE_DIR}/utils/libcxx/header_information.py"
   COMMENT "Generate the mapping file for include-what-you-use"
 )
 list(APPEND _all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")

libcxx/include/__cxx03/CMakeLists.txt

@@ -1037,7 +1037,7 @@ endforeach()
 # "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 ${_all_includes}
+  DEPENDS "${LIBCXX_SOURCE_DIR}/utils/libcxx/header_information.py"
   COMMENT "Generate the mapping file for include-what-you-use"
 )
 list(APPEND _all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")

libcxx/utils/libcxx/header_information.py

@@ -6,9 +6,9 @@
 #
 # ===----------------------------------------------------------------------===##
 
-import os, pathlib, functools
+import pathlib, functools
 
-libcxx_root = pathlib.Path(os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__)))))
+libcxx_root = pathlib.Path(__file__).resolve().parent.parent.parent
 libcxx_include = libcxx_root / "include"
 assert libcxx_root.exists()