Use type system entities instead of strings for node identity (#35918)

Use type system entities instead of strings for node identity (#35918)

The calling method for interface dispatches is used to provide imports specific to each call site (actually currently a shared one per calling method). Replace the use of mangled calling method name with its `MethodDesc` object as the key for the interface dispatch cell.

This saves ~4% working set measured on a large application.

Also, removed mangled name as the key for method-specific readonly data blobs the JIT allocates. This was much more modest since most methods don't need a readonly blob.