Remove librustdoc dependency on log

This change should have been included in PR #46386.

Since librustdoc doesn't explicitly depend on internal crates
(such as librustc_driver) through its Cargo.toml, it ends up using the
sysroot to resolve them. By removing the dependency on 'log',
we ensure that the syroot is used to resolve `log` as well. This ensures
that only one version of log is in use, so that `env_logger::init()`
enables all uses of `log!` macros.

Rename cross(2) builder to dist-various-{1,2}

Follows the convention of the other builders.

Report a targeted note for generic parameters that are missing a trait bound

Move rustc_back modules where they belong.

This change should have been included in PR #46386.

Since librustdoc doesn't explicitly depend on internal crates
(such as librustc_driver) through its Cargo.toml, it ends up using the
sysroot to resolve them. By removing the dependency on 'log',
we ensure that the syroot is used to resolve `log` as well. This ensures
that only one version of log is in use, so that `env_logger::init()`
enables all uses of `log!` macros.

incr.comp.: Incorporate the stable commandline arg hash and SVHs of upstream crates into the SVH.

So far the SVH detected changes in the HIR, which is already very sensitive, but in order for `eval_always` queries to also be sensitive to changes in upstream crates, the SVH also needs to capture changes there.

This PR fixes [rust-icci/crossbeam](https://travis-ci.org/rust-icci/crossbeam/builds/308936448), but I have not yet been able to come up with a minimal regression test.

r? @nikomatsakis

NLL: improve inference with flow results, represent regions with bitsets, and more

This PR begins with a number of edits to the NLL code and then includes a large number of smaller refactorings (these refactorings ought not to change behavior). There are a lot of commits here, but each is individually simple. The goal is to land everything up to but not including the changes to how we handle closures, which are conceptually more complex.

The NLL specific changes are as follows (in order of appearance):

**Modify the region inferencer's approach to free regions.** Previously, for each free region (lifetime parameter) `'a`, it would compute the set of other free regions that `'a` outlives (e.g., if we have `where 'a: 'b`, then this set would be `{'a, 'b}`). Then it would mark those free regions as "constants" and report an error if inference tried to extend `'a` to include any other region (e.g., `'c`) that is not in that outlives set. In this way, the value of `'a` would never grow beyond the maximum that could type check. The new approach is to allow `'a` to grow larger. Then, after the fact, we check over the value of `'a` and see what other free regions it is required to outlive, and we check that those outlives relationships are justified by the where clauses in scope etc.

**Modify constraint generation to consider maybe-init.** When we have a "drop-live" variable `x` (i.e., a variable that will be dropped but will not be otherwise used), we now consider whether `x` is "maybe initialized" at that point. If not, then we know the drop is a no-op, and we can allow its regions to be dead. Due to limitations in the fragment code, this currently only works at the level of entire variables.

**Change representation of regions to use a `BitMatrix`.** We used to use a `BTreeSet`, which was rather silly. We now use a MxN matrix of bits, where `M` is the number of variables and `N` is the number of possible elements in each set (size of the CFG + number of free regions).

The remaining commits (starting from
extract the `implied_bounds` code into a helper function ") are all "no-op" refactorings, I believe.

~~One concern I have is with the commit "with -Zverbose, print all details of closure substs"; this commit seems to include some "internal" stuff in the mir-dump files, such as internal interner numbers, that I fear may vary by platform. Annoying. I guess we will see.~~ (I removed this commit.)

As for reviewer, @arielb1 has been reviewing the PRs, and they are certainly welcome to review this one too. But I figured it'd maybe be good to have more people taking a look and being familiar with this code, so I'll "nominate" @pnkfelix .

r? @pnkfelix

Follows the convention of the other builders.

Fix invalid docs path for compiler plugins

The path to the docs `src/doc/guide-plugin.md` moved to
`src/doc/unstable-book/src/language-features/plugin.md`.

This patch updates it in the comment of WARNING message of the test
code.

This has been bugging me. All the regions appear free in the source;
the real difference is that some of them are universally quantified
(those in the function signature) and some are existentially
quantified (those for which we are inferring values).

Previously the code was somewhat duplicated.

No reason for it to live on `Inherited`.

We now visit just the stuff in the CFG, and we add liveness
constraints for all the random types, regions etc that appear within
rvalues and statements.

Now it can be reused by the NLL code.

This revealed some shortcomings, one of which is fixed. Fixes #45937.