[libc] Revamp hdrgen command line and build integration

This adds a new main command-line entry point for hdrgen, in the
new main.py.  This new interface is used for generating a header.
The old ways of invoking yaml_to_classes.py for other purposes
are left there for now, but `--e` is renamed to `--entry-point`
for consistency with the new CLI.

The YAML schema is expanded with the `header_template` key where
the corresponding `.h.def` file's path is given relative to where
the YAML file is found.  The build integration no longer gives
the `.h.def` path on the command line.  Instead, the script now
emits a depfile that's used by the cmake rules to track that.
The output file is always explicit in the script command line
rather than sometimes being derived from a directory path.

Author: frobtech

libc/cmake/modules/LLVMLibCHeaderRules.cmake

@@ -75,7 +75,7 @@ function(add_gen_header target_name)
   cmake_parse_arguments(
     "ADD_GEN_HDR"
     "PUBLIC" # No optional arguments
-    "YAML_FILE;DEF_FILE;GEN_HDR" # Single value arguments
+    "YAML_FILE;GEN_HDR" # Single value arguments
     "DEPENDS"     # Multi value arguments
     ${ARGN}
   )
@@ -84,9 +84,6 @@ function(add_gen_header target_name)
     add_library(${fq_target_name} INTERFACE)
     return()
   endif()
-  if(NOT ADD_GEN_HDR_DEF_FILE)
-    message(FATAL_ERROR "`add_gen_hdr` rule requires DEF_FILE to be specified.")
-  endif()
   if(NOT ADD_GEN_HDR_GEN_HDR)
     message(FATAL_ERROR "`add_gen_hdr` rule requires GEN_HDR to be specified.")
   endif()
@@ -97,8 +94,11 @@ function(add_gen_header target_name)
   set(absolute_path ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_GEN_HDR_GEN_HDR})
   file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_SOURCE_DIR} ${absolute_path})
   set(out_file ${LIBC_INCLUDE_DIR}/${relative_path})
+  set(dep_file "${out_file}.d")
+  file(RELATIVE_PATH rel_out_file ${CMAKE_BINARY_DIR} ${out_file})
+  file(RELATIVE_PATH rel_dep_file ${CMAKE_BINARY_DIR} ${dep_file})
   set(yaml_file ${CMAKE_SOURCE_DIR}/${ADD_GEN_HDR_YAML_FILE})
-  set(def_file ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_GEN_HDR_DEF_FILE})
+  file(RELATIVE_PATH rel_yaml_file ${CMAKE_BINARY_DIR} ${yaml_file})
 
   set(fq_data_files "")
   if(ADD_GEN_HDR_DATA_FILES)
@@ -108,26 +108,27 @@ function(add_gen_header target_name)
   endif()
 
   set(entry_points "${TARGET_ENTRYPOINT_NAME_LIST}")
-  list(TRANSFORM entry_points PREPEND "--e=")
+  list(TRANSFORM entry_points PREPEND "--entry-point=")
 
-  set(LIBC_HDRGEN "${LIBC_SOURCE_DIR}/utils/hdrgen/yaml_to_classes.py")
   add_custom_command(
     OUTPUT ${out_file}
-    COMMAND ${Python3_EXECUTABLE} ${LIBC_HDRGEN}
-            ${yaml_file}
-            --h_def_file ${def_file}
+    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
+    COMMAND ${Python3_EXECUTABLE} "${LIBC_SOURCE_DIR}/utils/hdrgen/main.py"
+            --output ${rel_out_file}
+            --depfile ${rel_dep_file}
             ${entry_points}
-            --output_dir ${out_file}
-    DEPENDS ${yaml_file} ${def_file} ${fq_data_files}
-    COMMENT "Generating header ${ADD_GEN_HDR_GEN_HDR} from ${yaml_file} and ${def_file}"
+            ${rel_yaml_file}
+    DEPENDS ${yaml_file} ${fq_data_files}
+    DEPFILE ${dep_file}
+    COMMENT "Generating header ${ADD_GEN_HDR_GEN_HDR} from ${yaml_file}"
   )
   if(LIBC_TARGET_OS_IS_GPU)
     file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/llvm-libc-decls)
     file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/llvm-libc-decls/gpu)
     set(decl_out_file ${LIBC_INCLUDE_DIR}/llvm-libc-decls/${relative_path})
     add_custom_command(
       OUTPUT ${decl_out_file}
-      COMMAND ${Python3_EXECUTABLE} ${LIBC_HDRGEN}
+      COMMAND ${Python3_EXECUTABLE} "${LIBC_SOURCE_DIR}/utils/hdrgen/yaml_to_classes.py"
               ${yaml_file}
               --export-decls
               ${entry_points}