[clang] Add missing header guard in <cpuid.h>

[mcgrathr]

Summary

This header has long lacked a standard multiple inclusion guard
like other headers have, for no apparent reason. The GCC header
of the same name likewise lacks one up through release 10.1, but
trunk GCC (release 11, and perhaps future 10.x) has fixed it
(see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96238).

[phosek]

LGTM

clang/lib/Headers/cpuid.h

 /*===---- cpuid.h - X86 cpu model detection --------------------------------===
  *
  * 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 __CPUID_H
+#define __CPUID_H
+
 #if !(__x86_64__ || __i386__)
 #error this header is for x86 only
 #endif
 
 /* Responses identification request with %eax 0 */
 /* AMD:     "AuthenticAMD" */
 #define signature_AMD_ebx 0x68747541
 #define signature_AMD_edx 0x69746e65
static __inline int __get_cpuid_count (unsigned int __leaf,
     unsigned int __max_leaf = __get_cpuid_max(__leaf & 0x80000000, 0);
 
     if (__max_leaf == 0 || __max_leaf < __leaf)
         return 0;
 
     __cpuid_count(__leaf, __subleaf, *__eax, *__ebx, *__ecx, *__edx);
     return 1;
 }
+
+#endif /* __CPUID_H */

clang/test/Headers/cpuid.c

 // RUN: %clang_cc1 %s -ffreestanding -triple x86_64-apple-darwin -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-64
 // RUN: %clang_cc1 %s -ffreestanding -triple i386 -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-32
 
 #include <cpuid.h>
+#include <cpuid.h> // Make sure multiple inclusion protection works.
 
 // CHECK-64: {{.*}} call { i32, i32, i32, i32 } asm "  xchgq  %rbx,${1:q}\0A cpuid\0A xchgq %rbx,${1:q}", "={ax},=r,={cx},={dx},0,~{dirflag},~{fpsr},~{flags}"(i32 %{{[a-z0-9]+}})
 // CHECK-64: {{.*}} call { i32, i32, i32, i32 } asm "  xchgq  %rbx,${1:q}\0A  cpuid\0A  xchgq  %rbx,${1:q}", "={ax},=r,={cx},={dx},0,2,~{dirflag},~{fpsr},~{flags}"(i32 %{{[a-z0-9]+}}, i32 %{{[a-z0-9]+}})
 
 // CHECK-32: {{.*}} call { i32, i32, i32, i32 } asm "cpuid", "={ax},={bx},={cx},={dx},0,~{dirflag},~{fpsr},~{flags}"(i32 %{{[a-z0-9]+}})
 // CHECK-32: {{.*}} call { i32, i32, i32, i32 } asm "cpuid", "={ax},={bx},={cx},={dx},0,2,~{dirflag},~{fpsr},~{flags}"(i32 %{{[a-z0-9]+}}, i32 %{{[a-z0-9]+}})
 
 unsigned eax0, ebx0, ecx0, edx0;
 unsigned eax1, ebx1, ecx1, edx1;
 
 void test_cpuid(unsigned level, unsigned count) {
   __cpuid(level, eax1, ebx1, ecx1, edx1);
   __cpuid_count(level, count, eax0, ebx0, ecx0, edx0);
 }