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

lib/system/repr_v2.nim

@@ -1,3 +1,5 @@
+include system/inclrtl
+
 proc isNamedTuple(T: typedesc): bool {.magic: "TypeTrait".}
   ## imported from typetraits
 
@@ -66,6 +68,11 @@ proc repr*[Enum: enum](x: Enum): string {.magic: "EnumToStr", noSideEffect.}
   ## If a `repr` operator for a concrete enumeration is provided, this is
   ## used instead. (In other words: *Overwriting* is possible.)
 
+proc reprDiscriminant*(e: int): string {.compilerproc.} =
+  # repr and reprjs can use `PNimType` to symbolize `e`; making this work here
+  # would require a way to pass the set of enum stringified values to cgen.
+  $e
+
 proc repr*(p: pointer): string =
   ## repr of pointer as its hexadecimal value
   if p == nil: