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

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).

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D91226

clang/lib/Headers/cpuid.h

@@ -7,6 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
+#ifndef __CPUID_H
+#define __CPUID_H
+
 #if !(__x86_64__ || __i386__)
 #error this header is for x86 only
 #endif
@@ -312,3 +315,5 @@ static __inline int __get_cpuid_count (unsigned int __leaf,
     __cpuid_count(__leaf, __subleaf, *__eax, *__ebx, *__ecx, *__edx);
     return 1;
 }
+
+#endif /* __CPUID_H */

clang/test/Headers/cpuid.c

@@ -2,6 +2,7 @@
 // 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]+}})