[ifs] IFSStub destructor should be virtual

This patch makes IFSStub class's destructor virtual to avoid
undefined behavior when it points to its child class's object and
got deleted.

Differential Revision: https://reviews.llvm.org/D154776

Author: Amirreza Ashouri

llvm/include/llvm/InterfaceStub/IFSStub.h

@@ -97,6 +97,7 @@ struct IFSStub {
   IFSStub() = default;
   IFSStub(const IFSStub &Stub);
   IFSStub(IFSStub &&Stub);
+  virtual ~IFSStub() = default;
 };
 
 // Create a alias class for IFSStub.