ARC related bugfixes and refactorings (#12781)

tests/destructor/texceptions.nim

@@ -0,0 +1,29 @@
+discard """
+  cmd: '''nim c --gc:arc $file'''
+  output: '''0'''
+"""
+
+proc other =
+  raise newException(ValueError, "stuff happening")
+
+proc indirectViaProcCall =
+  var correct = 0
+  for i in 1 .. 20:
+    try:
+      other()
+    except:
+      let x = getCurrentException()
+      correct += ord(x of ValueError)
+  doAssert correct == 20
+
+proc direct =
+  for i in 1 .. 20:
+    try:
+      raise newException(ValueError, "stuff happening")
+    except ValueError:
+      discard
+
+let mem = getOccupiedMem()
+indirectViaProcCall()
+direct()
+echo getOccupiedMem() - mem