[libc] Fix hdrgen output for no-argument functions (llvm#123245)

The hdrgen output is C, not C++.

Author: frobtech

libc/utils/hdrgen/function.py

@@ -22,7 +22,7 @@ def __init__(
 
     def __str__(self):
         attributes_str = " ".join(self.attributes)
-        arguments_str = ", ".join(self.arguments)
+        arguments_str = ", ".join(self.arguments) if self.arguments else "void"
         if attributes_str == "":
             result = f"{self.return_type} {self.name}({arguments_str})"
         else:

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

@@ -28,10 +28,10 @@ enum {
 
 __BEGIN_C_DECLS
 
-CONST_FUNC_A void func_a() __NOEXCEPT;
+CONST_FUNC_A void func_a(void) __NOEXCEPT;
 
 #ifdef LIBC_TYPES_HAS_FLOAT128
-float128 func_b() __NOEXCEPT;
+float128 func_b(void) __NOEXCEPT;
 #endif // LIBC_TYPES_HAS_FLOAT128
 
 #ifdef LIBC_TYPES_HAS_FLOAT16