Make rustc_mir::dataflow module pub (for clippy)

I'm working on fixing false-positives of a MIR-based clippy lint (rust-lang/rust-clippy#4509), and in need of the dataflow infrastructure.

Fix false-positive of redundant_clone and move to clippy::perf

This PR introduces dataflow analysis to `redundant_clone` lint to filter out borrowed variables, which had been incorrectly detected.

Depends on rust-lang/rust#64207.

changelog: Moved `redundant_clone` lint to `perf` group

# What this lint catches

## `clone`/`to_owned`

```rust
let s = String::new();
let t = s.clone();
```

```rust
// MIR
_1 = String::new();
_2 = &_1;
_3 = clone(_2); // (*)
```

We can turn this `clone` call into a move if

1. `_2` is the sole borrow of `_1` at the statement `(*)`
2. `_1` is not used hereafter

## `Deref` + type-specific `to_owned` method

```rust
let s = std::path::PathBuf::new();
let t = s.to_path_buf();
```

```rust
// MIR
_1 = PathBuf::new();
_2 = &1;
_3 = call deref(_2);
_4 = _3;                         // Copies borrow
StorageDead(_2);
_5 = Path::to_path_buf(_4); // (*)
```

We can turn this `to_path_buf` call into a move if

1. `_3` `_4` are the sole borrow of `_1` at `(*)`
2. `_1` is not used hereafter

# What this PR introduces

1. `MaybeStorageLive` that determines whether a local lives at a particular location
2. `PossibleBorrowerVisitor` that constructs [`TransitiveRelation`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/transitive_relation/struct.TransitiveRelation.html) of possible borrows, e.g. visiting `_2 = &1; _3 = &_2:` will result in `_3 -> _2 -> _1` relation. Then `_3` and `_2` will be counted as possible borrowers of `_1` in the sole-borrow analysis above.

Fix false-positive of redundant_clone and move to clippy::perf

This PR introduces dataflow analysis to `redundant_clone` lint to filter out borrowed variables, which had been incorrectly detected.

Depends on rust-lang/rust#64207.

changelog: Moved `redundant_clone` lint to `perf` group

# What this lint catches

## `clone`/`to_owned`

```rust
let s = String::new();
let t = s.clone();
```

```rust
// MIR
_1 = String::new();
_2 = &_1;
_3 = clone(_2); // (*)
```

We can turn this `clone` call into a move if

1. `_2` is the sole borrow of `_1` at the statement `(*)`
2. `_1` is not used hereafter

## `Deref` + type-specific `to_owned` method

```rust
let s = std::path::PathBuf::new();
let t = s.to_path_buf();
```

```rust
// MIR
_1 = PathBuf::new();
_2 = &1;
_3 = call deref(_2);
_4 = _3;                         // Copies borrow
StorageDead(_2);
_5 = Path::to_path_buf(_4); // (*)
```

We can turn this `to_path_buf` call into a move if

1. `_3` `_4` are the sole borrow of `_1` at `(*)`
2. `_1` is not used hereafter

# What this PR introduces

1. `MaybeStorageLive` that determines whether a local lives at a particular location
2. `PossibleBorrowerVisitor` that constructs [`TransitiveRelation`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/transitive_relation/struct.TransitiveRelation.html) of possible borrows, e.g. visiting `_2 = &1; _3 = &_2:` will result in `_3 -> _2 -> _1` relation. Then `_3` and `_2` will be counted as possible borrowers of `_1` in the sole-borrow analysis above.

Rollup of 2 pull requests

Successful merges:

 - #4509 (Fix false-positive of redundant_clone and move to clippy::perf)
 - #4614 (Allow casts from the result of `abs` to unsigned)

Failed merges:

r? @ghost

Rollup of 2 pull requests

Successful merges:

 - #4509 (Fix false-positive of redundant_clone and move to clippy::perf)
 - #4614 (Allow casts from the result of `abs` to unsigned)

Failed merges:

changelog: none

r? @ghost