SI-9356 more careful assertion in back-end

Calling `exists` on a `Symbol` triggers unpickling,
which failed for reasons I did not investigate.

Replaced `sym.exists` by `sym != NoSymbol`, which is good enough here.
Also replaced assertion by a `devWarning`, since the
logic seems too ad-hoc to actually crash the compiler when it's invalidated.

Partially reverts b45a91fe22. See also #1532.

1 files changed, 6 insertions, 0 deletions

diff --git a/test/files/pos/t9356/Foo_2.scala b/test/files/pos/t9356/Foo_2.scala
new file mode 100644
index 0000000000..ab7bb44d0e
--- /dev/null
+++ b/test/files/pos/t9356/Foo_2.scala
@@ -0,0 +1,6 @@
+class C
+
+trait Foo {
+  @annot.MyAnnotation(cls = classOf[C])
+  def function: Any = ???
+}