fix #19426 compile error using when/elif/else and typedesc in template (#20550)

tests/whenstmt/t19426.nim

@@ -0,0 +1,16 @@
+type
+  MyInt = object
+    bitWidth: int
+
+template toRealType*(t: MyInt): typedesc =
+  when t.bitWidth == 32: int32
+  elif t.bitWidth == 64: int64
+  else: {.error.}
+
+proc doFail(T: typedesc): T = default(T)
+
+proc test =
+  const myInt = MyInt(bitWidth:32)
+  discard doFail(toRealType(myInt))
+
+test()