Fix for #21456 (Regressions in attribute allocations for non-generic attributes) (#21462)
* Fix for #21456 - restrict increased generic attribute allocations to only generic attributes
This is a trivial quick-fix for #21456 where regressions between 2.1 and 3.0 were discovered on most attibute pathways due to the allocation overhead in the generic-supporting pathways. The workaround is to simply not take that slow/expensive path for non-generics.
While I'd like to optimize `RuntimeModule.ResolveMethod` further, there's a public surface area in play there that makes the changes non-trivial. There, we'll have to choose overhead on the public path (which may still be a net win), or duplication in code for another path.
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