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.

This commit removes the `dep_graph` field from the `Session` type according to
issue #44390. Most of the fallout here was relatively straightforward and the
`prepare_session_directory` function was rejiggered a bit to reuse the results
in the later-called `load_dep_graph` function.

Closes #44390

Remove the `cstore` reference from Session in order to prepare encapsulating CrateStore access in tcx.

This should now be entirely tracked through queries, so no need to have a
`DepGraph` in the `CStore` object any more!

Fixes #41701.

inc comp: -Z profile-queries support; see also https://github.com/rust-lang-nursery/rust-forge/blob/master/profile-queries.md

Like #43008 (f6689991), but _much more aggressive_.

Bizarrely, librustc_passes, librustc_plugin, librustc_mir, and libsyntax
weren't getting their error explanations registered.

Resolves #35284.

Replaces the output path env var. Can be passed to save-analysis via a function call or env var.

This PR is an implementation of [RFC 1974] which specifies a new method of
defining a global allocator for a program. This obsoletes the old
`#![allocator]` attribute and also removes support for it.

[RFC 1974]: https://github.com/rust-lang/rfcs/pull/197

The new `#[global_allocator]` attribute solves many issues encountered with the
`#![allocator]` attribute such as composition and restrictions on the crate
graph itself. The compiler now has much more control over the ABI of the
allocator and how it's implemented, allowing much more freedom in terms of how
this feature is implemented.

cc #27389

Prior to this PR, when we aborted because a "critical pass" failed, we
displayed the number of errors from that critical pass. While that's the
number of errors that caused compilation to abort in *that place*,
that's not what people really want to know. Instead, always report the
total number of errors, and don't bother to track the number of errors
from the last pass that failed.

This changes the compiler driver API to handle errors more smoothly,
and therefore is a compiler-api-[breaking-change].

Fixes #42793.

This reverts commit 5558c64f.