[NFC] Add optional include to Format.h

Format.h uses std::optional but doesn't include the optional header
resulting in a build failure due to a missing 'optional' type.

Reviewed By: compnerd, mib

Differential Revision: https://reviews.llvm.org/D154664

Author: etcwilde

llvm/include/llvm/Support/Format.h

@@ -28,6 +28,7 @@
 #include "llvm/Support/DataTypes.h"
 #include <cassert>
 #include <cstdio>
+#include <optional>
 #include <tuple>
 #include <utility>