clang/win: Add __readfsdword to intrin.h

[thakis]

Summary

When using __readfsdword(), clang used to warn that one has
to include <intrin.h> -- no matter if that was already included
or not.

Now it only warns if it's not yet included.

To verify that this was the only intrin with this problem, I ran:

$ for f in $(grep intrin.h clang/include/clang/Basic/BuiltinsX86* |
             egrep -o '\([^,]+,' | egrep -o '[^(,]*'); do
    if ! grep -q $f clang/lib/Headers/intrin.h; then echo $f; fi;
  done

This printed 9 more functions, but those are all in emmintrin.h,
xsaveintrin.h (which are included by intrin.h based on /arch: flags).
So this is indeed the only built-in that was missing in intrin.h.

Fixes PR51188.

[thakis]

no-op rebase in the hope that the precommit bot doesn't run in a "git lockfile already held" error again

[hans]

lgtm

clang/lib/Headers/intrin.h

 unsigned long __readcr0(void);
 unsigned long __readcr2(void);
 unsigned __LPTRINT_TYPE__ __readcr3(void);
 unsigned long __readcr4(void);
 unsigned long __readcr8(void);
 unsigned int __readdr(unsigned int);
 #ifdef __i386__
 unsigned char __readfsbyte(unsigned long);
-unsigned __int64 __readfsqword(unsigned long);
 unsigned short __readfsword(unsigned long);
+unsigned long __readfsdword(unsigned long);
+unsigned __int64 __readfsqword(unsigned long);
 #endif
 unsigned __int64 __readmsr(unsigned long);
 unsigned __int64 __readpmc(unsigned long);
 unsigned long __segmentlimit(unsigned long);
 void __sidt(void *);
 void __stosb(unsigned char *, unsigned char, size_t);
 void __stosd(unsigned long *, unsigned long, size_t);
 void __stosw(unsigned short *, unsigned short, size_t);

clang/test/CodeGen/X86/ms-x86-intrinsics.c

-// RUN: %clang_cc1 -ffreestanding -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
+// RUN: %clang_cc1 -Werror -ffreestanding -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
 // RUN:         -triple i686--windows -Oz -emit-llvm %s -o - \
 // RUN:         | FileCheck %s -check-prefix=CHECK -check-prefix=CHECK-I386
-// RUN: %clang_cc1 -ffreestanding -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
+// RUN: %clang_cc1 -Werror -ffreestanding -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
 // RUN:         -triple x86_64--windows -Oz -emit-llvm %s -o - \
 // RUN:         | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-X64
 
+#include <intrin.h>
+
 #if defined(__i386__)
 char test__readfsbyte(unsigned long Offset) {
   return __readfsbyte(++Offset);
 }
 // CHECK-I386-LABEL: define dso_local signext i8 @test__readfsbyte(i32 %Offset)
 // CHECK-I386:   %inc = add i32 %Offset, 1
 // CHECK-I386:   [[PTR:%[0-9]+]] = inttoptr i32 %inc to i8 addrspace(257)*
 // CHECK-I386:   [[VALUE:%[0-9]+]] = load volatile i8, i8 addrspace(257)* [[PTR]], align 1