fix semcase on tySequence and tyObject #20283 #19682 (#20339)

* fix semcase on tySequence and tyObject #20283 #19682

* use better arg name

* avoiding returns nil use errorNode instead, clean code

* use efNoDiagnostics flag

* remove tests/errmsgs/t19682.nim

* combine 2 test cases to one file

tests/errmsgs/tcase_stmt.nim

@@ -0,0 +1,29 @@
+discard """
+  cmd: "nim check --hints:off $file"
+  errormsg: ""
+  nimout: '''
+tcase_stmt.nim(22, 7) Error: selector must be of an ordinal type, float or string
+tcase_stmt.nim(28, 6) Error: selector must be of an ordinal type, float or string
+
+
+
+
+
+
+
+'''
+"""
+
+
+
+# bug #19682
+type A = object
+
+case A()
+else:
+  discard
+
+# bug #20283
+
+case @[]
+else: discard