gdb::optional unit tests

I thought I'd add some unit tests to make sure gdb::optional behaved
correctly, and started writing some, but then thought/realized that
libstdc++ already has extensive testing for C++17 std::optional, which
gdb::optional is a subset of, and thought why bother writing something
from scratch.  So I tried copying over a subset of libstdc++'s tests
(that ones that cover the subset supported by gdb::optional), and was
positively surprised that they mostly work OOTB.  This did help shake
out a few bugs from what I was implementing in the previous patch to
gdb::optional.  Still, it's a good chunk of code being copied over, so
if people dislike this copying/duplication, I can drop this patch.

gdb/ChangeLog:
2017-04-18  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/optional-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add optional-selftests.o.
	* unittests/optional-selftests.c: New file.
	* unittests/optional/assignment/1.cc: New file.
	* unittests/optional/assignment/2.cc: New file.
	* unittests/optional/assignment/3.cc: New file.
	* unittests/optional/assignment/4.cc: New file.
	* unittests/optional/assignment/5.cc: New file.
	* unittests/optional/assignment/6.cc: New file.
	* unittests/optional/assignment/7.cc: New file.
	* unittests/optional/cons/copy.cc: New file.
	* unittests/optional/cons/default.cc: New file.
	* unittests/optional/cons/move.cc: New file.
	* unittests/optional/cons/value.cc: New file.
	* unittests/optional/in_place.cc: New file.
	* unittests/optional/observers/1.cc: New file.
	* unittests/optional/observers/2.cc: New file.

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