Begin converting uses of PyCallable to PythonCallable.

PyCallable is a class that exists solely within the swig wrapper
code.  PythonCallable is a more generic implementation of the same
idea that can be used by any Python-related interop code, and lives
in PythonDataObjects.h

The CL is mostly mechanical, and it doesn't cover every possible
user of PyCallable, because I want to minimize the impact of this
change (as well as making it easier to figure out what went wrong
in case this causes a failure).  I plan to finish up the rest of
the changes in a subsequent patch, culminating in the removal of
PyCallable entirely.

llvm-svn: 252906