[libc] Use response files for hdrgen entry points (llvm#183267)

This avoids the build failure when the command line ends up being too
long which can be especially problematic on Windows.

Fixes llvm#182374

Author: petrhosek

libc/cmake/modules/LLVMLibCHeaderRules.cmake

@@ -109,7 +109,10 @@ function(add_gen_header target_name)
     set(entry_points "${TARGET_ENTRYPOINT_NAME_LIST}")
   endif()
 
+  # TODO: Use $<LIST:PREPEND,list,item,...> after we bump CMake to 3.27.
   list(TRANSFORM entry_points PREPEND "--entry-point=")
+  set(rsp_file "${CMAKE_CURRENT_BINARY_DIR}/${relative_path}.rsp")
+  file(GENERATE OUTPUT ${rsp_file} CONTENT "$<JOIN:${entry_points},\n>")
 
   add_custom_command(
     OUTPUT ${out_file}
@@ -119,9 +122,9 @@ function(add_gen_header target_name)
             --depfile ${dep_file}
             --write-if-changed
             ${proxy_arg}
-            ${entry_points}
             ${yaml_file}
-    DEPENDS ${yaml_file}
+            "@${rsp_file}"
+    DEPENDS ${yaml_file} ${rsp_file}
     DEPFILE ${dep_file}
     COMMENT "Generating header ${ADD_GEN_HDR_GEN_HDR} from ${yaml_file}"
   )

libc/utils/hdrgen/hdrgen/main.py

@@ -18,7 +18,9 @@
 
 
 def main():
-    parser = argparse.ArgumentParser(description="Generate header files from YAML")
+    parser = argparse.ArgumentParser(
+        description="Generate header files from YAML", fromfile_prefix_chars="@"
+    )
     parser.add_argument(
         "yaml_file",
         help="Path to the YAML file containing header specification",