eliminate nontrivial Reset(...) from TypedPythonObject

Summary:
This deletes Reset(...), except for the no-argument form Reset()
from TypedPythonObject, and therefore from PythonString, PythonList,
etc.

It updates the various callers to use assignment, As<>, Take<>,
and Retain<>, as appropriate.

followon to https://reviews.llvm.org/D69080

Reviewers: JDevlieghere, clayborg, labath, jingham

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69133

llvm-svn: 375350