gdb::optional: Add observers

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.