Fix crash in -stack-list-arguments

Fix crash in -stack-list-arguments

-stack-list-arguments will crash when stopped in an Ada procedure that
has an argument with a certain name ("_objectO" -- which can only be
generated by the compiler).  The bug occurs because lookup_symbol will
fail in this case.

This patch changes -stack-list-arguments to mirror what is done with
arguments elsewhere.  (As an aside, I don't understand why this lookup
is even needed, but I assume it is some stabs thing?)

In the longer term I think it would be good to share this code between
MI and the CLI.  However, due to the upcoming release, I preferred a
more local fix.

gdb/ChangeLog
2020-07-22  Tom Tromey  <tromey@adacore.com>

	* mi/mi-cmd-stack.c (list_args_or_locals): Use
	lookup_symbol_search_name.

gdb/testsuite/ChangeLog
2020-07-22  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/mi_prot.exp: New file.
	* gdb.ada/mi_prot/pkg.adb: New file.
	* gdb.ada/mi_prot/pkg.ads: New file.
	* gdb.ada/mi_prot/prot.adb: New file.