Enable unloading of AssemblyLoadContext (dotnet/coreclr#18476)

Enable unloading of AssemblyLoadContext (dotnet/coreclr#18476)

Enable assembly unloading

* Allow PInvoke methods on collectible assemblies

* Fix test unloadability

Several hundreds of tests were using Helper class that created
GCHandle, but never freed it. That prevented unloading of those
tests. The change modifies the Helper class to keep the handle
in a finalizable object.

Several GCHandle related tests were not freeing the GCHandle they
allocated, so this change adds freeing them to enable the unloading.

* Add missing error messages to the resources

* Fix shuffle thunk cache for unloadability

* Add GetLoaderAllocator to ICLRPrivBinder

Commit migrated from https://github.com/dotnet/coreclr/commit/8cd4b39a42c1c7cf37502357e6a4cb2888f5dfd7