With host directives we can end up in a situation where the same directive applies multiple times to the same element, potentially with conflicting configurations. The runtime isn't set up for a directive to apply more than once so historically we were throwing an error when we detect duplicates.
This ended up limiting the usefulness of host directives to library authors, because it meant that host directives couldn't be reused as much as authors wanted. To address the issue, these changes introduce logic in the compiler and runtime that will de-duplicate host directives with the following logic:
1. If a directive matches once in the template and more than once as a host directive, the host directive matches will be discarded and only the template match will apply. The mental model is that a host directive match represents `Partial<YourDirective>` while a template match represents the full `YourDirective`.
2. If a directive matches multiple times as a host directive, we merge the input/output mappings from all the instances into a single one. If we detect a case where an input/output is exposed under multiple names during the merging process, both the compiler and the runtime will produce an error.
Fixes#57846.
9c55fcb3e6