Subzero: Fix a memory leak.

The problem was that Translator and subclasses need to have virtual destructors since they are used as unique_ptr<>.  As a result, only the Translator base class destructor was being invoked.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4290
TEST= make -f Makefile.standalone ASAN=1 check-lit
R=ascull@google.com

Review URL: https://codereview.chromium.org/1281003003.

src/PNaClTranslator.h

diff --git a/src/PNaClTranslator.h b/src/PNaClTranslator.h
index b6286e8..24a627b 100644
--- a/src/PNaClTranslator.h
+++ b/src/PNaClTranslator.h
@@ -34,6 +34,7 @@
 
 public:
   explicit PNaClTranslator(GlobalContext *Ctx) : Translator(Ctx) {}
+  ~PNaClTranslator() override = default;
 
   /// Reads the PNaCl bitcode file and translates to ICE, which is then
   /// converted to machine code. Sets ErrorStatus to 1 if any errors