Add "operator bool" to SB APIs

Summary:
Our python version of the SB API has (the python equivalent of)
operator bool, but the C++ version doesn't.

This is because our python operators are added by modify-python-lldb.py,
which performs postprocessing on the swig-generated interface files.

In this patch, I add the "operator bool" to all SB classes which have an
IsValid method (which is the same logic used by modify-python-lldb.py).
This way, we make the two interfaces more constent, and it allows us to
rely on swig's automatic syntesis of python __nonzero__ methods instead
of doing manual fixups.

Reviewers: zturner, jingham, clayborg, jfb, serge-sans-paille

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D58792

llvm-svn: 355824