Fix build bug in ada-lang.c

Fix build bug in ada-lang.c

An earlier patch of mine introduced a build failure in ada-lang.c.  A
couple of "to_string" calls were not namespace-qualified.  In the
failing setup, the std string_view is being used, and so (apparently)
ADL doesn't find gdb::to_string.

This patch, from the bug, fixes the problem.

gdb/ChangeLog
2021-03-06  Chernov Sergey  <klen_s@mail.ru>

	PR gdb/27528:
	* ada-lang.c (ada_fold_name): Use gdb::to_string.