patch 8.0.0218: no completion for :cexpr and similar commands

Problem:    No command line completion for :cexpr, :cgetexpr, :caddexpr, etc.
Solution:   Make completion work. (Yegappan Lakshmanan)  Add a test.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 295e539..cff41cf 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4109,6 +4109,12 @@
 	case CMD_echoerr:
 	case CMD_call:
 	case CMD_return:
+	case CMD_cexpr:
+	case CMD_caddexpr:
+	case CMD_cgetexpr:
+	case CMD_lexpr:
+	case CMD_laddexpr:
+	case CMD_lgetexpr:
 	    set_context_for_expression(xp, arg, ea.cmdidx);
 	    break;