Fix #2662: Don't convert subtype typedesc params

There is no point to issue implicit HiddenStdConv encountering subtype of
typedesc[Base] parameter on overload resolution, since this will anyway never
reach codegen. This change effectively fixes compiler bug for:

  iterator it(T: typedesc[Base]) = ...
  for s in it(SubclassOfBase): ...

Where HiddenStdConv triggered implicit instantiation of variable of type
typedesc[Base] in for transform, that eventually fails at getUniqueType, that
refuses to work for typedesc.

compiler/sigmatch.nim

@@ -1281,7 +1281,10 @@ proc paramTypesMatchAux(m: var TCandidate, f, argType: PType,
     result = implicitConv(nkHiddenStdConv, f, arg, m, c)
   of isSubtype:
     inc(m.subtypeMatches)
-    result = implicitConv(nkHiddenSubConv, f, arg, m, c)
+    if f.kind == tyTypeDesc:
+      result = arg
+    else:
+      result = implicitConv(nkHiddenSubConv, f, arg, m, c)
   of isSubrange:
     inc(m.subtypeMatches)
     if f.kind == tyVar: