Make virtual the destructor for symex_complexity_limit_exceeded_actiont.

[markrtuttle]

The class symex_complexity_limit_exceeded_actiont has a virtual function transform and no virtual destructor.

On macos, the cmake build with clang fails with

[ 60%] Building CXX object src/goto-symex/CMakeFiles/goto-symex.dir/auto_objects.cpp.o
In file included from /Users/mrtuttle/cbmc/src/goto-symex/auto_objects.cpp:12:
In file included from /Users/mrtuttle/cbmc/src/goto-symex/goto_symex.h:15:
In file included from /Users/mrtuttle/cbmc/src/util/options.h:15:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/string:500:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/string_view:176:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/__string:56:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:644:
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1880:58: error: destructor called on non-final
      'symex_complexity_limit_exceeded_actiont' that has virtual functions but non-virtual destructor
      [-Werror,-Wdelete-non-virtual-dtor]
    _LIBCPP_INLINE_VISIBILITY void destroy(pointer __p) {__p->~_Tp();}
                                                         ^

[smowton]

Clang-format might complain (try it locally), otherwise lgtm

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 78a1bb0).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/132584397
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[markrtuttle]

@peterschrammel I fixed the clang-format problem. I think this is okay to merge now.