[silgen] Fix a small typo bug I found on inspection.

We are performing a cast instead of a dyn_cast in an if statement. This will
assert on failure rather than fall through. It seems to be an obvious typo. I
checked in the actual commit in question that introduced it and the intention
was to be a dyn_cast.

rdar://149698148

Author: gottesmm

lib/SILGen/ResultPlan.cpp

@@ -81,7 +81,7 @@ class IndirectOpenedSelfCleanup final : public Cleanup {
     assert(box && "buffer never emitted before activating cleanup?!");
     auto theBox = box;
     if (SGF.getASTContext().SILOpts.supportsLexicalLifetimes(SGF.getModule())) {
-      if (auto *bbi = cast<BeginBorrowInst>(theBox)) {
+      if (auto *bbi = dyn_cast<BeginBorrowInst>(theBox)) {
         SGF.B.createEndBorrow(loc, bbi);
         theBox = bbi->getOperand();
       }