Optional: add reset()

It is part of C++17

Change-Id: I2aa4a11133a3d77644f4819492623c8e424ccd59
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/3rdparty/optional/optional.hpp

@@ -433,6 +433,8 @@ public:
   // 20.5.4.2, Destructor
   ~optional() = default;
 
+  void reset() noexcept { clear(); }
+
   // 20.5.4.3, assignment
   optional& operator=(nullopt_t) noexcept
   {