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.
5 files changed