fortify: fix tests on x86_64

My declval hack failed on x86_64 with:

```
substitution failure [with T = __va_list_tag [1]]: function cannot
return array type '__va_list_tag [1]'
```

...Because the type of va_list is compiler magic, it's fine for it to be
whatever the compiler wants it to be. Thankfully, pointers to arrays can
be returned, so let's use those instead.

Bug: 135210098
Test: mma on aosp_blueline-eng and aosp_x86_64-eng; the latter was
      failing before this patch.

Change-Id: Iefd57c0f8e823653fd70633fb6ee75cfc0022430

tests/clang_fortify_tests.cpp

@@ -307,7 +307,7 @@ static void testFcntl() {
 static void testFormatStrings() {
   const auto unsigned_value = declval<unsigned long long>();
   const auto* unknown_string = declval<const char*>();
-  const auto va = declval<va_list>();
+  const auto va = *declval<va_list*>();
 
   {
     auto some_fd = declval<int>();