Don't show const value in error message

[Wh1teDuke]

Test

const FOO = 1
let   BAR = 2
add(FOO, BAR)

Expected
expression: add(FOO, BAR)

Got
expression: add(1, BAR)

Related: #6293

---

>nim -v
Nim Compiler Version 0.20.99 [Linux: i386]
Compiled at 2019-07-18
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: 4becd5a2a8d67c7481af457d5007e1a313d454c8
active boot switches: -d:release

[mratsim]

That seems complex, I suppose the VM is applying constant folding to replace const by their value ASAP instead of storing an ident.

[Araq]

Also affects Nim's tools like the docgen.

[timotheecour]

Also affects Nim's tools like the docgen.

indeed => #7039