Disallow enum with indirections

[dlangBugzillaToGithub]

Bolpat reported this on 2017-09-01T23:29:30Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17799

Description

It is easily overseen that using enums creates a new instance every time as enums usually have types without indirections or are immutable (e.g. strings).

  enum foo = [1,2,3];

results in

  assert(foo !is foo);

which is weird. Any other identifier that represents an array wouldn't have this oddity.
Make it hard to use incorrectly and disallow enums that have mutable indirections.

See https://forum.dlang.org/post/qgfdcibsepmewijgvaeb@forum.dlang.org