MathExtras: MulOverflow: use builtin when available (NFC) (#95046)

This matches the other two functions AddOverflow and SubOverflow.

Author: Superty

llvm/include/llvm/Support/MathExtras.h

@@ -635,6 +635,9 @@ std::enable_if_t<std::is_signed_v<T>, T> SubOverflow(T X, T Y, T &Result) {
 /// result, returning true if an overflow ocurred.
 template <typename T>
 std::enable_if_t<std::is_signed_v<T>, T> MulOverflow(T X, T Y, T &Result) {
+#if __has_builtin(__builtin_mul_overflow)
+  return __builtin_mul_overflow(X, Y, &Result);
+#else
   // Perform the unsigned multiplication on absolute values.
   using U = std::make_unsigned_t<T>;
   const U UX = X < 0 ? (0 - static_cast<U>(X)) : static_cast<U>(X);
@@ -656,6 +659,7 @@ std::enable_if_t<std::is_signed_v<T>, T> MulOverflow(T X, T Y, T &Result) {
     return UX > (static_cast<U>(std::numeric_limits<T>::max()) + U(1)) / UY;
   else
     return UX > (static_cast<U>(std::numeric_limits<T>::max())) / UY;
+#endif
 }
 
 } // namespace llvm