Add virtual destructor (necessary due to the switch to shared_ptr).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281198 91177308-0d34-0410-b5e6-96231b3b80d8

utils/TableGen/ClangDiagnosticsEmitter.cpp

@@ -911,6 +911,7 @@ namespace {
 struct DiagText {
   struct Piece {
     virtual void print(std::vector<std::string> &RST) = 0;
+    virtual ~Piece() {}
   };
   struct TextPiece : Piece {
     StringRef Role;