clang/win: Let the compiler know that NaClSwitch() cannot return.

src/trusted/service_runtime/arch/x86_32/nacl_switch_to_app_32.c

Index: src/trusted/service_runtime/arch/x86_32/nacl_switch_to_app_32.c
diff --git a/src/trusted/service_runtime/arch/x86_32/nacl_switch_to_app_32.c b/src/trusted/service_runtime/arch/x86_32/nacl_switch_to_app_32.c
index c6c4a5c502b75f870b291342a80bb771b679dd5c..47e4d9d9b800020842a33a8e510afa99eb7f38de 100644
--- a/src/trusted/service_runtime/arch/x86_32/nacl_switch_to_app_32.c
+++ b/src/trusted/service_runtime/arch/x86_32/nacl_switch_to_app_32.c
@@ -70,4 +70,12 @@ NORETURN void NaClStartThreadInApp(struct NaClAppThread *natp,
   context->sysret = 0; /* %eax not used to return */
 
   NaClSwitch(context);
+
+#if NACL_WINDOWS && defined(__clang__)
+  /*
+   * NaClSwitch is a function pointer in x86_32 and there's no way to mark
+   * the function pointed to by a function pointer as noreturn in clang-cl mode.
+   */
+  __builtin_unreachable();
+#endif
 }