Python gc fixes prototype

[rpoyner-tri]

This is the dev branch for fixes toward #14387 . I won't merge anything directly from here, but rather peel of merge-able PRs separately. This branch should always contain the remainder of the work, though not necessarily in proper form.

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This change is 

[rpoyner-tri]

Updated. Please note, as usual, that docs and tests are far from complete.

Reviewed 1 of 4 files at r2, 2 of 35 files at r4.
Reviewable status: needs platform reviewer assigned, needs at least two assigned reviewers, labeled "do not merge", commits need curation (https://drake.mit.edu/reviewable.html#curated-commits), missing label for release notes

[rpoyner-tri]

Reviewable status: 1 unresolved discussion, needs platform reviewer assigned, needs at least two assigned reviewers, labeled "do not merge", commits need curation (https://drake.mit.edu/reviewable.html#curated-commits), missing label for release notes

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bindings/pydrake/systems/framework_py_semantics.cc line 593 at r4 (raw file):

template <typename T>
struct BuilderLifeSupport {

This bit (and changes to drake/systems/framework) helps cover the rare, but probably unavoidable case of a program that adds systems via Python, but calls Build() from C++. See, for example, Anzu just before anzu#14558. Note that from-Python Build() calls abandon(), so that in the all-Python case there are no immortals (leaks), only collectible ref-cycles.

[rpoyner-tri]

Reviewable status: 1 unresolved discussion, needs platform reviewer assigned, needs at least two assigned reviewers, labeled "do not merge", commits need curation (https://drake.mit.edu/reviewable.html#curated-commits), missing label for release notes

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bindings/pydrake/systems/framework_py_semantics.cc line 593 at r4 (raw file):

Previously, rpoyner-tri (Rick Poyner (rico)) wrote…

This bit (and changes to drake/systems/framework) helps cover the rare, but probably unavoidable case of a program that adds systems via Python, but calls Build() from C++. See, for example, Anzu just before anzu#14558. Note that from-Python Build() calls abandon(), so that in the all-Python case there are no immortals (leaks), only collectible ref-cycles.

To be clear, the code this enables is the code that was deleted in anzu#14558.