add heuristics decreasing false possible "possible leaks" in c++ code.

The option --leak-check-heuristics=heur1,heur2,... can activate
various heuristics to decrease the number of false positive
"possible leaks" for C++ code. The available heuristics are
detecting valid interior pointers to std::stdstring, to new[] allocated
arrays with elements having destructors and to interior pointers pointing
to an inner part of a C++ object using multiple inheritance.

This fixes 280271 Valgrind reports possible memory leaks on still-reachable
std::string 

This has been tested on x86/amd64/ppc32/ppc64.

First performance measurements seems to show a neglectible impact on
the leak search.

More feedback welcome both on performance and functional aspects
(false positive 'possibly leaked' rate decrease and/or 
false negative 'possibly leaked' rate increase).

Note that the heuristic is not checking that the memory has been
allocated with "new" or "new[]", as it is expected that in some cases,
specific alloc fn are used for c++ objects instead of the standard new/new[].
If needed, we might add an option to check the alloc functions
to be new/new[].



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13582 a5019735-40e9-0310-863c-91ae7b9d1cf9