Extend indexing API interface to provide information about used
directives' selectors on template elements. This enables an indexer to
xref element attributes to the directives that match them.
The current way this matching is done is by mapping selectors to indexed
directives. However, this fails in cases where the directive is not
indexed by the indexer API, like for transitive dependencies. This
solution is much more general.
PR Close#31782
44039a4b16