[libc][hdrgen] Print __BEGIN_C_DECLS / __END_C_DECLS conditionally. (#188830)

Clean up the `%public_api` printer code slightly - get rid of explicit
`\n` and ensure we only print `__BEGIN_C_DECLS` and `__END_C_DECLS` if
the generated header actually contains functions or objects to declare.

I've noticed that after 27ba9e2a44 landed,
generated errno.h header has two blocks of `__BEGIN_C_DECLS` /
`__END_C_DECLS`: an empty one was generated automatically from
`%public_api` section that was intended to only add the `errno_t` type
declaration.

libc/utils/hdrgen/hdrgen/header.py

@@ -280,9 +280,14 @@ class HeaderFile:
             self.include_lines(self.template_file is None)
             + self.macro_lines()
             + self.enum_lines()
-            + ["\n__BEGIN_C_DECLS\n"]
         )
+        content.append("")
+        has_decls = self.functions or self.objects
+        if has_decls:
+            content.append("__BEGIN_C_DECLS")
+            content.append("")
 
+        # Emit function declarations.
         current_guard = None
         last_name = None
         for function in sorted(self.functions):
@@ -317,10 +322,13 @@ class HeaderFile:
             content.append(f"#endif // {current_guard}")
             content.append("")
 
+        # Emit object declarations.
         content.extend(str(object) for object in self.objects)
         if self.objects:
             content.append("")
-        content.append("__END_C_DECLS")
+
+        if has_decls:
+            content.append("__END_C_DECLS")
 
         return "\n".join(content)
 

libc/utils/hdrgen/tests/expected_output/macro_only.h

@@ -0,0 +1,16 @@
+//===-- Standard C header <macro_only.h> --===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LLVM_LIBC_MACRO_ONLY_H
+#define _LLVM_LIBC_MACRO_ONLY_H
+
+#include "__llvm-libc-common.h"
+
+#define MACRO_A 1
+
+#endif // _LLVM_LIBC_MACRO_ONLY_H

libc/utils/hdrgen/tests/input/macro_only.yaml

@@ -0,0 +1,6 @@
+header: macro_only.h
+standards:
+  - stdc
+macros:
+  - macro_name: MACRO_A
+    macro_value: 1

libc/utils/hdrgen/tests/test_integration.py

@@ -82,13 +82,20 @@ class TestHeaderGenIntegration(unittest.TestCase):
         self.run_script(yaml_file, output_file)
         self.compare_files(output_file, expected_output_file)
 
-    def test_generate_header(self):
+    def test_generate_proxy_header(self):
         yaml_file = self.source_dir / "input/test_small.yaml"
         expected_output_file = self.source_dir / "expected_output/test_small_proxy.h"
         output_file = self.output_dir / "test_small.h"
         self.run_script(yaml_file, output_file, switches=["--proxy"])
         self.compare_files(output_file, expected_output_file)
 
+    def test_generate_macro_only_header(self):
+        yaml_file = self.source_dir / "input/macro_only.yaml"
+        expected_output_file = self.source_dir / "expected_output/macro_only.h"
+        output_file = self.output_dir / "macro_only.h"
+        self.run_script(yaml_file, output_file)
+        self.compare_files(output_file, expected_output_file)
+
 def main():
     parser = argparse.ArgumentParser(description="TestHeaderGenIntegration arguments")
     parser.add_argument(