Handle PyLong return values in LLDBSwigPython_CalculateNumChildren.

Summary:
Also, change its return type to size_t to match the return types of
its callers.

With this change, std::vector and std::list data formatter tests
pass on Linux (when using libstdc++) with clang as well as with gcc.
These tests have also been enabled in this patch.

Test Plan: dotest.py -p <TestDataFormatterStdVector|TestDataFormatterStdList>

Reviewers: vharron, clayborg

Reviewed By: clayborg

Subscribers: zturner, lldb-commits

Differential Revision: http://reviews.llvm.org/D8337

llvm-svn: 232399