include generic bodies in allowMetaTypes (#23968)

fixes #19848

Not sure why this wasn't the case already. The `if cl.allowMetaTypes:
return` line below for `tyFromExpr` [was added 10 years
ago](d5798b43de).
Hopefully it was just negligence?

(cherry picked from commit 1befb8d4a3)

tests/generics/t19848.nim

@@ -0,0 +1,16 @@
+discard """
+  output: '''
+todo
+'''
+"""
+
+type
+  Maybe[T] = object
+  List[T] = object
+
+proc dump[M: Maybe](a: List[M]) =
+  echo "todo"
+
+var a: List[Maybe[int]]
+  
+dump(a)