[libc][NFC] Simplify how the definiton of NULL macro is added to string.h.

We do not list macro definitions in api.td files anymore. Not all macros
definitions have been moved out. This change moves the definition of the
NULL macro out.

Reviewed By: lntue, jhuber6

Differential Revision: https://reviews.llvm.org/D140376

Author: Siva Chandra Reddy

libc/config/gpu/api.td

@@ -2,17 +2,6 @@ include "config/public_api.td"
 
 include "spec/stdc.td"
 
-def NullMacro : MacroDef<"NULL"> {
-  let Defn = [{
-    #define __need_NULL
-    #include <stddef.h>
-  }];
-}
-
 def StringAPI : PublicAPI<"string.h"> {
   let Types = ["size_t"];
-
-  let Macros = [
-    NullMacro,
-  ];
 }

libc/config/linux/api.td

@@ -36,13 +36,6 @@ def StaticAssertMacro : MacroDef<"static_assert"> {
   }];
 }
 
-def NullMacro : MacroDef<"NULL"> {
-  let Defn = [{
-    #define __need_NULL
-    #include <stddef.h>
-  }];
-}
-
 def AssertAPI : PublicAPI<"assert.h"> {
   let Macros = [
     AssertMacro,
@@ -134,10 +127,6 @@ def FenvAPI: PublicAPI<"fenv.h"> {
 
 def StringAPI : PublicAPI<"string.h"> {
   let Types = ["size_t"];
-
-  let Macros = [
-    NullMacro,
-  ];
 }
 
 def StdIOAPI : PublicAPI<"stdio.h"> {

libc/include/CMakeLists.txt

@@ -91,6 +91,7 @@ add_gen_header(
   GEN_HDR string.h
   DEPENDS
     .llvm_libc_common_h
+    .llvm-libc-macros.null_macro
     .llvm-libc-types.size_t
 )
 

libc/include/llvm-libc-macros/CMakeLists.txt

@@ -6,6 +6,12 @@ add_header(
     generic-error-number-macros.h
 )
 
+add_header(
+  null_macro
+  HDR
+    null-macro.h
+)
+
 add_header(
   fcntl_macros
   HDR

libc/include/llvm-libc-macros/null-macro.h

@@ -0,0 +1,15 @@
+//===-- Definition of the NULL macro --------------------------------------===//
+//
+// 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_MACROS_NULL_MACRO_H
+#define __LLVM_LIBC_MACROS_NULL_MACRO_H
+
+#define __need_NULL
+#include <stddef.h>
+
+#endif // __LLVM_LIBC_MACROS_NULL_MACRO_H

libc/include/string.h.def

@@ -11,6 +11,8 @@
 
 #include <__llvm-libc-common.h>
 
+#include <llvm-libc-macros/null-macro.h>
+
 %%public_api()
 
 #endif // LLVM_LIBC_STRING_H