Rollup merge of #123362 - oli-obk:thread_local_nested_statics, r=estebank

Check that nested statics in thread locals are duplicated per thread.

follow-up to #123310

cc ``@compiler-errors`` ``@RalfJung``

fwiw: I have no idea how thread local statics make that work under LLVM, and miri fails on this example, which I would have expected to be the correct behaviour.

Since the `#[thread_local]` attribute is just an internal implementation detail, I'm just going to start hard erroring on nested mutable statics in thread locals.