Pass void_context_p to parse_expression

An earlier patch pointed out that nothing in GDB sets void_context_p
when parsing an expression.  This patch fixes this omission.

"print" and "call" differ in that the former will print a value that
has void type, while the latter will not.  AdaCore has had a patch for
a long time that uses this distinction to help with overload
resolution.  In particular, in a "call" context, a procedure will be
chosen, while in a "print" context, a zero-argument function will be
chosen instead.

Regression tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-01-08  Tom Tromey  <tromey@adacore.com>

	* parse.c (parse_expression): Add void_context_p parameter.  Use
	parse_exp_in_context.
	* printcmd.c (print_command_1): Change voidprint to bool.  Pass to
	parse_expression.
	(print_command, call_command): Update.
	* expression.h (parse_expression): Add void_context_p parameter.

gdb/testsuite/ChangeLog
2021-01-08  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/voidctx/pck.adb: New file.
	* gdb.ada/voidctx/pck.ads: New file.
	* gdb.ada/voidctx/voidctx.adb: New file.
	* gdb.ada/voidctx.exp: New file.