fixes #20515; base method requires explicit {.gcsafe.} to be GC-safe (#20574)

* fixes #20515; base requires explicit `{.gcsafe.}` to be GC-safe

* add tests

tests/method/t20515.nim

@@ -0,0 +1,20 @@
+discard """
+  errormsg: "Base method 'zzz' requires explicit '{.gcsafe.}' to be GC-safe"
+  line: 10
+"""
+
+type
+  A = ref object of RootObj
+  B = ref object of A
+
+method zzz(a: A) {.base.} =
+  discard
+
+var s: seq[int]
+method zzz(a: B) =
+  echo s
+
+proc xxx(someObj: A) {.gcsafe.} =
+  someObj.zzz()
+
+xxx(B())