Fix for dotnet/coreclr#21456 (Regressions in attribute allocations for non-generic...

Fix for dotnet/coreclr#21456 (Regressions in attribute allocations for non-generic attributes) (dotnet/coreclr#21462)

* Fix for dotnet/coreclr#21456 - restrict increased generic attribute allocations to only generic attributes

This is a trivial quick-fix for dotnet/coreclr#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.

* Update comments

Commit migrated from https://github.com/dotnet/coreclr/commit/130cec3b15815ae023a7aac9dc96aa97b36117a0