[lld] [LLD][COFF] Introduce Symbol::getDefined helper. (NFC) (PR #151253)

[Jacek Caban via llvm-commits]

================
@@ -91,6 +91,14 @@ bool Symbol::isLive() const {
   return true;
 }
 
+Defined *Symbol::getDefined() {
+  if (auto d = dyn_cast<Defined>(this))
+    return d;
+  if (auto u = dyn_cast<Undefined>(this))
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cjacek wrote:

Yes, I believe the assumption that only one of them can succeed is safe. I wrote it in this order because I’d expect `Defined` to be more common, but as you said, the performance impact is not really relevant.

https://github.com/llvm/llvm-project/pull/151253