fixed dot operator recursive loop & macro suggest (#16922)

* basic stability improvements; refs nimsuggest
* fixed dot operator recursive loop & macro suggest
* hacky fix for run away dot operator sem check

Committing this mostly to make the issue more clear. Perhaps get better
feedback.

* semExprWithType seems like a better place to check
* fixed error messages const case expressions
* Clean-up test
* stopped the dot operator madness

No longer get infinite recursion when seming broken code with a dot
operator macro like in jsffi.

Co-authored-by: Araq <rumpf_a@web.de>

compiler/vmgen.nim

@@ -505,7 +505,10 @@ proc genCase(c: PCtx; n: PNode; dest: var TDest) =
       let it = n[i]
       if it.len == 1:
         # else stmt:
-        c.gen(it[0], dest)
+        if it[0].kind != nkNilLit or it[0].typ != nil:
+          # an nkNilLit with nil for typ implies there is no else branch, this
+          # avoids unused related errors as we've already consumed the dest
+          c.gen(it[0], dest)
       else:
         let b = rawGenLiteral(c, it)
         c.gABx(it, opcBranch, tmp, b)