Enable warnings inside system headers

Author: MaxEW707

clang/test/Headers/ms-intrin.cpp

@@ -1,31 +1,31 @@
 // RUN: %clang_cc1 -triple i386-pc-win32 -target-cpu pentium4 \
 // RUN:     -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
-// RUN:     -ffreestanding -fsyntax-only -Werror \
+// RUN:     -ffreestanding -fsyntax-only -Werror -Wsystem-headers \
 // RUN:     -isystem %S/Inputs/include %s
 
 // RUN: %clang_cc1 -triple i386-pc-win32 -target-cpu broadwell \
 // RUN:     -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
-// RUN:     -ffreestanding -emit-obj -o /dev/null -Werror \
+// RUN:     -ffreestanding -emit-obj -o /dev/null -Werror -Wsystem-headers \
 // RUN:     -isystem %S/Inputs/include %s
 
 // RUN: %clang_cc1 -triple x86_64-pc-win32  \
 // RUN:     -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
-// RUN:     -ffreestanding -emit-obj -o /dev/null -Werror \
+// RUN:     -ffreestanding -emit-obj -o /dev/null -Werror -Wsystem-headers \
 // RUN:     -isystem %S/Inputs/include %s
 
 // RUN: %clang_cc1 -triple thumbv7--windows \
 // RUN:     -fms-compatibility -fms-compatibility-version=17.00 \
-// RUN:     -ffreestanding -fsyntax-only -Werror \
+// RUN:     -ffreestanding -fsyntax-only -Werror -Wsystem-headers \
 // RUN:     -isystem %S/Inputs/include %s
 
 // RUN: %clang_cc1 -triple aarch64--windows \
 // RUN:     -fms-compatibility -fms-compatibility-version=17.00 \
-// RUN:     -ffreestanding -fsyntax-only -Werror \
+// RUN:     -ffreestanding -fsyntax-only -Werror -Wsystem-headers \
 // RUN:     -isystem %S/Inputs/include %s
 
 // RUN: %clang_cc1 -triple arm64ec--windows \
 // RUN:     -fms-compatibility -fms-compatibility-version=17.00 \
-// RUN:     -ffreestanding -fsyntax-only -Werror \
+// RUN:     -ffreestanding -fsyntax-only -Werror -Wsystem-headers \
 // RUN:     -isystem %S/Inputs/include %s
 
 // REQUIRES: x86-registered-target