Previously some checkers attached explicitly created program point tags
to some of the exploded graph nodes that they created. In most of the
checkers this ad-hoc tagging only affected the debug dump of the
exploded graph (and they weren't too relevant for debugging) so this
commit removes them.
There were two checkers where the tagging _did_ have a functional role:
- In `RetainCountChecker` the presence of tags were checked by
`RefCountReportVisitor`.
- In `DynamicTypePropagation` the checker sometimes wanted to create two
identical nodes and had to apply an explicit tag on the second one to
avoid "caching out".
In these two situations I preserved the tags but switched to using
`SimpleProgramPointTag` instead of `CheckerProgramPointTag` because
`CheckerProgramPointTag` didn't provide enough benefits to justify its
existence.
Note that this commit depends on the earlier commit "[analyzer] Fix
tagging of PostAllocatorCall" ec96c0c072
and would introduce crashes when cherry-picked onto a branch that
doesn't contain that commit.
For more details about the background see the discourse thread
https://discourse.llvm.org/t/role-of-programpointtag-in-the-static-analyzer/
As a tangentially related changes, this commit also adds some comments
to document the surprising behavior of `CheckerContext::addTransition`
and an assertion in the constructor of `PathSensitiveBugReport` to get a
more readable crash dump in the case when the report is constructed with
`nullptr` as the `ErrorNode`. (This can happen due to "caching out".)
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