Warn if add-symbol-file does not provide any symbols

A user suggested that add-symbol-file ought to warn if the file does
not in fact provide any symbols.  This seemed like a decent idea, so
this patch implements this idea.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* symfile.c (add_symbol_file_command): Remove obsolete comment.
	Warn if symbol file does not provide any symbols.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* gdb.base/symfile-warn.exp: New file.
	* gdb.base/symfile-warn.c: New file.