Add completion for Ada catch commands

Add completion for Ada catch commands

This patch adds a completion function to the "catch exception"
and "catch handlers" commands.

Tested on x86-64 Fedora 29; reviewed off-list by Joel.

gdb/ChangeLog
2019-05-10  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (catch_ada_completer): New function.
	(_initialize_ada_language): Use it.

gdb/testsuite/ChangeLog
2019-05-10  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/info_exc.exp: Add "complete" test.