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.
Otherwise we may emit double errors related to the `#[export_name]` attribute,
for example, and using a query should ensure that it's only emitted at most
once.
This commit moves the `collect_and_partition_translation_items` function into a
query on `TyCtxt` instead of a free function in trans, allowing us to track
dependencies and such of the function.
