This commit alters how we compile LLVM by default enabling the WebAssembly
backend. This then also adds the wasm32-unknown-unknown target to get compiled
on the `cross` builder and distributed through rustup. Tests are not yet enabled
for this target but that should hopefully be coming soon!

No functional changes intended.

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 attaches a channel to the LLVM workers to the `TyCtxt` which will
later be used during the codegen query to actually send work to LLVM workers.
Otherwise this commit is just plumbing this channel throughout the compiler to
ensure it reaches the right consumers.

This commit moves the definition of the `ExportedSymbols` structure to the
`rustc` crate and then creates a query that'll be used to construct the
`ExportedSymbols` set. This in turn uses the reachablity query exposed in the
previous commit.

Turns out this was already set up as a query, just wasn't using it yet!

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

This commit removes the `cstore_untracked` method, making the `CrateStore` trait
object entirely private to the `ty/context.rs` module.

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

This commit primarily removes the `stability` field from `TyCtxt` as well as its
internal mutable state, instead using a query to build the stability index as
well as primarily using queries for other related lookups.

Like previous commits the calculation of the stability index is wrapped in a
`with_ignore` node to avoid regressing the current tests, and otherwise this
commit also introduces #44232 but somewhat intentionally so.

This commit moves the calculation of the `LanguageItems` structure into a
query rather than being calculated before the `TyCtxt` exists, with the eventual
end goal of removing some `CrateStore` methods.

This commit started by moving methods from `CrateStore` to queries, but it ended
up necessitating some deeper refactorings to move more items in general to
queries.

Before this commit the *resolver* would walk over the AST and process foreign
modules (`extern { .. }` blocks) and collect `#[link]` annotations. It would
then also process the command line `-l` directives and such. This information
was then stored as precalculated lists in the `CrateStore` object for iterating
over later.

After this, commit, however, this pass no longer happens during resolution but
now instead happens through queries. A query for the linked libraries of a crate
will crawl the crate for `extern` blocks and then process the linkage
annotations at that time.

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