[Support] Simplify has_StreamOperator (NFC) (llvm#159242)

Without this patch, we are doing a roundtrip on types.  Specifically,
if decltype(...) is well formed, std::is_same_v evaluates to a boolean
value.  We then pass the boolean value to std::enable_if_t, go through
the sizeof(char)/sizeof(double) trick, and then come back to a boolean
value.

This patch simplifies all this by having test() return
std::is_same<...>.  The "caller" attaches ::value, so effectively we
are using std::is_same<...>::value when decltype(...) is well formed,
bypassing std::enable_if_t and the sizeof(char)/sizeof(double) trick.

If we did not care about the return type of the shift operator, we
could use llvm::is_detected, but the return type check doesn't allow
us to simplify things that far.

Author: kazutakahirata

llvm/include/llvm/Support/FormatVariadicDetails.h

@@ -78,15 +78,14 @@ template <class T> class has_StreamOperator {
   using ConstRefT = const std::decay_t<T> &;
 
   template <typename U>
-  static char test(std::enable_if_t<
-                   std::is_same_v<decltype(std::declval<llvm::raw_ostream &>()
-                                           << std::declval<U>()),
-                                  llvm::raw_ostream &>,
-                   int *>);
+  static auto test(int)
+      -> std::is_same<decltype(std::declval<llvm::raw_ostream &>()
+                               << std::declval<U>()),
+                      llvm::raw_ostream &>;
 
-  template <typename U> static double test(...);
+  template <typename U> static auto test(...) -> std::false_type;
 
-  static bool const value = (sizeof(test<ConstRefT>(nullptr)) == 1);
+  static constexpr bool value = decltype(test<ConstRefT>(0))::value;
 };
 
 // Simple template that decides whether a type T should use the member-function