properly fix #10053 ; FieldDefect msg now shows discriminant value + lineinfo, in all backends (c,vm,js) (#11955)

* fix #10053 FieldError for vm

* fixup

* FieldError now also shows runtime value of discriminant

* fix field error reporting in vm

* also report culprit line info in err msg

* fix errors for newruntime 2

* fix for js

* fixup

* PRTEMP4

* works

* works

* works perfect

* refactor

* std/private/repr_impl

* suppport --gc:arc

* cleanup

* refactor

* simplify

* simplify

* simplify

* fixup

* move out compiler.vmgen.genCustom

* fixup

* fixup

* add tests

* revert compiler/debugutils.nim

* simplify reprDiscriminant

* fixup

* lib/std/private/repr_impl.nim -> lib/system/repr_impl.nim

* try to fix D20210812T165220

* honor --declaredlocs

* control toFileLineCol via --declaredlocs

compiler/cgen.nim

@@ -15,7 +15,7 @@ import
   ccgutils, os, ropes, math, passes, wordrecg, treetab, cgmeth,
   rodutils, renderer, cgendata, aliases,
   lowerings, tables, sets, ndi, lineinfos, pathutils, transf,
-  injectdestructors
+  injectdestructors, astmsgs
 
 when not defined(leanCompiler):
   import spawn, semparallel