Rollup merge of #146778 - nikic:allocator-shim-attributes, r=jackh726

Use standard attribute logic for allocator shim

Use llfn_attrs_from_instance() to generate the attributes for the allocator shim. This ensures that we generate all the usual attributes (and don't get to find out one-by-one that a certain attribute is important for a certain target). Additionally this will enable emitting the allocator-specific attributes (not included here).

This change is quite awkward because the allocator shim uses SimpleCx, while llfn_attrs_from_instance uses CodegenCx. I've switched it to use SimpleCx plus tcx/sess arguments where necessary. If there's a simpler way to do this, I'd love to know about it...

Author: matthiaskrgr