Smarter variant object construction (#11273)

tests/objvariant/trt_discrim_err2.nim

@@ -0,0 +1,14 @@
+discard """
+  errormsg: "branch initialization with a runtime discriminator only supports ordinal types with 2^16 elements or less."
+  line: 13
+"""
+type
+  HoledObj = object
+    case kind: int
+    of 0: a: int
+    else: discard
+
+let someInt = low(int)
+case someInt
+of 938: echo HoledObj(kind: someInt, a: 1)
+else: discard