effects: exclude swap() from "indirect calls" assumption (#15504)

swap() will never call any procs passed to it, and so it can be safely
excluded from the "assume indirect calls are taken" effects tracking
rule.

tests/effects/teffects10.nim

@@ -0,0 +1,12 @@
+discard """
+action: compile
+"""
+
+# https://github.com/nim-lang/Nim/issues/15495
+
+proc f() {.raises: [].} =
+  var a: proc ()
+  var b: proc ()
+  swap(a, b)
+
+f()