Add LLVM_ATTRIBUTE_USED to DominanceInfo::dump

This function was unavailable in the debugger because it was getting
optimized away.

Author: meg-gupta

include/swift/SIL/Dominance.h

@@ -93,7 +93,7 @@ class DominanceInfo : public DominatorTreeBase {
   }
 
 #ifndef NDEBUG
-  void dump() { print(llvm::errs()); };
+  void dump() LLVM_ATTRIBUTE_USED { print(llvm::errs()); }
 #endif
 };