Add regression test: 3C inserts syntactically invalid casts into `valist.c` on Windows

[mattmccutchen-cci]

3C inserts syntactically invalid casts to __builtin_va_list & into valist.c on Windows:

--- valist.c    2021-02-21 11:13:09.584208800 -0500
+++ valist.checked.c    2021-02-25 08:52:06.325658600 -0500
@@ -1,29 +1,29 @@
 // RUN: rm -rf %t*
 // RUN: 3c -base-dir=%S -alltypes -addcr %s -- | FileCheck -match-full-lines -check-prefixes="CHECK_ALL","CHECK" %s
 // RUN: 3c -base-dir=%S -addcr %s -- | FileCheck -match-full-lines -check-prefixes="CHECK_NOALL","CHECK" %s
 // RUN: 3c -base-dir=%S -addcr %s -- | %clang -c -fcheckedc-extension -x c -o /dev/null -
 // RUN: 3c -base-dir=%S -output-dir=%t.checked -alltypes %s --
 // RUN: 3c -base-dir=%t.checked -alltypes %t.checked/valist.c -- | diff %t.checked/valist.c -
 
 #include <stdarg.h>
 typedef int lua_State;
 extern void lua_lock(lua_State *);
 extern void luaC_checkGC(lua_State *);
 extern void lua_unlock(lua_State *);
 extern const char *luaO_pushvfstring (lua_State *L, const char *fmt, va_list argp);
-const char *lua_pushfstring (lua_State *L, const char *fmt, ...) {
+const char *lua_pushfstring(lua_State *L : itype(_Ptr<lua_State>), const char *fmt : itype(_Ptr<const char>), ...) : itype(_Ptr<const char>) {
        //CHECK: const char *lua_pushfstring(lua_State *L : itype(_Ptr<lua_State>), const char *fmt : itype(_Ptr<const char>), ...) : itype(_Ptr<const char>) {
   const char *ret;
        //CHECK: const char *ret;
   va_list argp;
   lua_lock(L);
-  va_start(argp, fmt);
+  va_start(((__builtin_va_list & )argp), fmt);
   ret = luaO_pushvfstring(L, fmt, argp);
-  va_end(argp);
+  va_end(((__builtin_va_list & )argp));
   luaC_checkGC(L);
   lua_unlock(L);
   return ret;
 }
 /*force output*/
-int *p;
+_Ptr<int> p = ((void *)0);
        //CHECK: _Ptr<int> p = ((void *)0);

I haven't investigated why that is happening or what __builtin_va_list & even means.

[john-h-kastner]

Once issue #459 is fixed, the test case for that issue will also act as a regression test for this issue that can be run on linux.