rustc: Move codegen to a query

This commit moves the actual code generation in the compiler behind a query
keyed by a codegen unit's name. This ended up entailing quite a few internal
refactorings to enable this, along with a few cut corners:

* The `OutputFilenames` structure is now tracked in the `TyCtxt` as it affects a
  whole bunch of trans and such. This is now behind a query and threaded into
  the construction of the `TyCtxt`.

* The `TyCtxt` now has a channel "out the back" intended to send data to worker
  threads in rustc_trans. This is used as a sort of side effect of the codegen
  query but morally what's happening here is the return value of the query
  (currently unit but morally a path) is only valid once the background threads
  have all finished.

* Dispatching work items to the codegen threads was refactored to only rely on
  data in `TyCtxt`, which mostly just involved refactoring where data was
  stored, moving it from the translation thread to the controller thread's
  `CodegenContext` or the like.

* A new thread locals was introduced in trans to work around the query
  system. This is used in the implementation of `assert_module_sources` which
  looks like an artifact of the old query system and will presumably go away
  once red/green is up and running.

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