Style variable names

Style variable names

This adds style support for variable names.  For the time being, this
is only done in backtraces, not in ptype or print; those places do not
use ui-out and so would need ad hoc changes.

This also adds styling to the names printed for local variables in
"backtrace full".  This code does not use ui-out, so the styling is
done using the low-level API.

gdb/ChangeLog
2018-12-28  Tom Tromey  <tom@tromey.com>

	* ui-out.h (enum class ui_out_style_kind) <VARIABLE>: New global.
	* stack.c (print_frame_arg): Style name.
	* printcmd.c (print_variable_and_value): Style variable name.
	* cli/cli-style.h (variable_name_style): Declare.
	* cli/cli-style.c (variable_name_style): New global.
	(_initialize_cli_style): Update.
	* cli-out.c (cli_ui_out::do_field_string): Update.

gdb/testsuite/ChangeLog
2018-12-28  Tom Tromey  <tom@tromey.com>

	* gdb.base/style.exp: Add test for variable names.