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