* Value gets renamed to Operand, so that now interpret::{Place, Operand} are the
  "dynamic" versions of mir::{Place, Operand}.
* Operand and Place share the data for their "stuff is in memory"-base in a new
  type, MemPlace.  This also makes it possible to give some more precise types
  in other areas.  Both Operand and MemPlace have methods available to project
  into fields (and other kinds of projections) without causing further
  allocations.
* The type for "a Scalar or a ScalarPair" is called Value, and again used to
  give some more precise types.
* All of these have versions with an attached layout, so that we can more often
  drag the layout along instead of recomputing it.  This lets us get rid of
  `PlaceExtra::Downcast`.  MPlaceTy and PlaceTy can only be constructed
  in place.rs, making sure the layout is handled properly.
  (The same should eventually be done for ValTy and OpTy.)
* All the high-level functions to write typed memory take a Place, and live in
  place.rs.  All the high-level typed functions to read typed memory take an
  Operand, and live in operands.rs.

This commit upgrades the main LLVM submodule to LLVM's current master branch.
The LLD submodule is updated in tandem as well as compiler-builtins.

Along the way support was also added for LLVM 7's new features. This primarily
includes the support for custom section concatenation natively in LLD so we now
add wasm custom sections in LLVM IR rather than having custom support in rustc
itself for doing so.

Some other miscellaneous changes are:

* We now pass `--gc-sections` to `wasm-ld`
* The optimization level is now passed to `wasm-ld`
* A `--stack-first` option is passed to LLD to have stack overflow always cause
  a trap instead of corrupting static data
* The wasm target for LLVM switched to `wasm32-unknown-unknown`.
* The syntax for aligned pointers has changed in LLVM IR and tests are updated
  to reflect this.
* The `thumbv6m-none-eabi` target is disabled due to an [LLVM bug][llbug]

Nowadays we've been mostly only upgrading whenever there's a major release of
LLVM but enough changes have been happening on the wasm target that there's been
growing motivation for quite some time now to upgrade out version of LLD. To
upgrade LLD, however, we need to upgrade LLVM to avoid needing to build yet
another version of LLVM on the builders.

The revision of LLVM in use here is arbitrarily chosen. We will likely need to
continue to update it over time if and when we discover bugs. Once LLVM 7 is
fully released we can switch to that channel as well.

[llbug]: https://bugs.llvm.org/show_bug.cgi?id=37382

This new query returns only the predicates *directly defined* on an
item (in contrast to the more common `predicates_of`, which returns
the predicates that must be proven to reference an item). These two
sets are almost always identical except for traits, where
`predicates_of` includes an artificial `Self: Trait<...>` predicate
(basically saying that you cannot use a trait item without proving
that the trait is implemented for the type parameters).

This new query is only used in chalk lowering, where this artificial
`Self: Trait` predicate is problematic. We encode it in metadata but
only where needed since it is kind of repetitive with existing
information.
Co-authored-by: NTyler Mandry <tmandry@gmail.com>

This avoids a decent number of allocations, enough to speed up
incremental runs of many rustc-benchmarks, the best by 2%.

Fixes #50501

This computes the transitive closure of traits that appear in the
environment and then appends their clauses. It needs some work, but
it's in the right direction.

Fixes #49690

Implement inferring outlives requirements for references, structs, enum, union, and projection types. added a feature gate and tests for these scenarios.

resolving it as a dependency.

Fixes #46816

In addition to the `Fn*` family of traits, closures now implement `Copy` (and similarly `Clone`) if all of the captures do.