gdb::optional: Add observers

Currently, gdb::optional is really minimal and can only be used for
lazy initialization.  There's no way to get at the value contained
inside the optinal.  This commit corrects that, by adding observer
methods, mostly copied from libstdc++'s implementation of C++17
std::optional.

This will be used in the following patch.

gdb/ChangeLog:
2017-04-04  Pedro Alves  <palves@redhat.com>

	* common/gdb_optional.h (gdb::optiona): Add operator->, operator*,
	operator bool, has_value and get methods.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2017-04-04  Pedro Alves  <palves@redhat.com>
+
+	* common/gdb_optional.h (gdb::optiona): Add operator->, operator*,
+	operator bool, has_value and get methods.
+
 2017-04-04  Pedro Alves  <palves@redhat.com>
 
 	* dwarf2read.c (struct file_entry): Add ctors, and initialize all

gdb/common/gdb_optional.h

@@ -61,6 +61,31 @@ public:
     m_instantiated = true;
   }
 
+  /* Observers.  */
+  constexpr const T *operator-> () const
+  { return std::addressof (this->get ()); }
+
+  T *operator-> ()
+  { return std::addressof (this->get ()); }
+
+  constexpr const T &operator* () const &
+  { return this->get (); }
+
+  T &operator* () &
+  { return this->get (); }
+
+  T &&operator* () &&
+  { return std::move (this->get ()); }
+
+  constexpr const T &&operator* () const &&
+  { return std::move (this->get ()); }
+
+  constexpr explicit operator bool () const noexcept
+  { return m_instantiated; }
+
+  constexpr bool has_value () const noexcept
+  { return m_instantiated; }
+
 private:
 
   /* Destroy the object.  */
@@ -71,6 +96,10 @@ private:
     m_item.~T ();
   }
 
+  /* The get operations have m_instantiated as a precondition.  */
+  T &get () noexcept { return m_item; }
+  constexpr const T &get () const noexcept { return m_item; }
+
   /* The object.  */
   union
   {