Fix broken cache invalidation with deallocated and reallocated document node

The original caching implementation had an oversight in combination with
the new lifetime management in DOM f

Fix broken cache invalidation with deallocated and reallocated document node

The original caching implementation had an oversight in combination with
the new lifetime management in DOM for 8.3.
The modification counter is stored on the document object itself, but as
that can get deallocated when all references disappear, stale cache data
can be used. Normally this isn't a problem, unless getElementsByTagName is
called not on the document but on a child node. Fix it by moving caching
data into the ref object, which will outlive all nodes from a document
even if the document object disappears.

Closes GH-12338.

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