Remove some ui_out-related cleanups from Python

This patch introduces a bit of infrastructure -- namely, a minimal
std::optional analogue called gdb::optional, and an RAII template
class that works like make_cleanup_ui_out_tuple_begin_end or
make_cleanup_ui_out_list_begin_end -- and then uses these in the
Python code.  This removes a number of cleanups and generally
simplifies this code.

std::optional is only available in C++17.  Normally I would have had
this code check __cplusplus, but my gcc apparently isn't new enough to
find <optional>, even with -std=c++1z; so, because I could not test
it, the patch does not do this.

gdb/ChangeLog
2017-02-10  Tom Tromey  <tom@tromey.com>

	* ui-out.h (ui_out_emit_type): New class.
	(ui_out_emit_tuple, ui_out_emit_list): New typedefs.
	* python/py-framefilter.c (py_print_single_arg): Use gdb::optional
	and ui_out_emit_tuple.
	(enumerate_locals): Likewise.
	(py_mi_print_variables, py_print_locals, py_print_args): Use
	ui_out_emit_list.
	(py_print_frame): Use gdb::optional, ui_out_emit_tuple,
	ui_out_emit_list.
	* common/gdb_optional.h: New file.

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