Refactor the optimization passes to explicitly use the passes. This commit
just re-implements the same passes as were already being run.

It also adds an option (behind `-Z`) to run the LLVM lint pass on the
unoptimized IR.

Fix for #6575. In the trans phase, rustc emits code for a function parameter that goes completely unused in the event the return type of the function in question happens to be an immediate.

This patch modifies rustc & parts of rustrt to ensure that the vestigial parameter is no longer present in compiled code.

Also as a bonus this fixes #6767

it can alias `&const` and `@mut`

it can alias `&const` and `@mut`

I think settings like this should be left up to the user.

RIP `alt`.

This works with pandoc linked against highlighting-kate >= 0.5.3.8. It seems to just be a no-op with earlier versions, because I successfully ran this through `try`.

This also fixes some consistency issues (like making `Example`/`Examples` always a header and always using three tildes).

The compiler guarantees that there are no other references to a unique pointer when it's passed by-value to a function.

The existence of the header and annihilator don't matter since it's not relevant to the call:

> For a call to the parent function, dependencies between memory references from before or after the call and from those during the call are “irrelevant” to the noalias keyword for the arguments and return value used in that call.

@graydon's tracing garbage collector stores the metadata outside of the boxes, so that won't be a problem. I'm unsure if updating the header while inside a function where it's marked as `noalias` would be a problem anyway since you never actually read or write to the header.

@nikomatsakis: r?

r? @thestinger As called for in #4994

Per the recommendation of the now-removed FIXME.

Via my super-accurate timing (`/usr/bin/time -l`), I get 2431119360 => 1884430336 == ~520MB

Closes #6637.

Smallintmap was being populated with sparse keys during ast deserialization.

* Add a short section and an example illustrating the use of ARC. 
* Header for the section of Future changed to be more descriptive: "Backgrounding computations: Futures".

Shaves off ~600MB of memory while compiling rustc

Smallintmap was being populated with sparse keys during ast deserialization.

r? @brson

Apple Clang uses different version numbering than "regular" clang, but
it also provides the "regular" version it's based on. Update the sed
pattern to pull out this "regular" version number instead of the Apple
version number.

Passing higher alignment values gives the optimization passes more freedom since it can copy in larger chunks. This change results in rustc outputting the same post-optimization IR as clang for swaps and most copies excluding the lack of information about padding.

Code snippet:

```rust
#[inline(never)]
fn swap<T>(x: &mut T, y: &mut T) {
    util::swap(x, y);
}
```

Original IR (for `int`):

```llvm
define internal fastcc void @_ZN9swap_283417_a71830ca3ed2d65d3_00E(i64*, i64*) #1 {
static_allocas:
  %2 = icmp eq i64* %0, %1
  br i1 %2, label %_ZN4util9swap_283717_a71830ca3ed2d65d3_00E.exit, label %3

; <label>:3                                       ; preds = %static_allocas
  %4 = load i64* %0, align 1
  %5 = load i64* %1, align 1
  store i64 %5, i64* %0, align 1
  store i64 %4, i64* %1, align 1
  br label %_ZN4util9swap_283717_a71830ca3ed2d65d3_00E.exit

_ZN4util9swap_283717_a71830ca3ed2d65d3_00E.exit:  ; preds = %3, %static_allocas
  ret void
}
```

After #6710:

```llvm
define internal fastcc void @_ZN9swap_283017_a71830ca3ed2d65d3_00E(i64* nocapture, i64* nocapture) #1 {
static_allocas:
  %2 = load i64* %0, align 1
  %3 = load i64* %1, align 1
  store i64 %3, i64* %0, align 1
  store i64 %2, i64* %1, align 1
  ret void
}
```

After this change:

```llvm
define internal fastcc void @_ZN9swap_283017_a71830ca3ed2d65d3_00E(i64* nocapture, i64* nocapture) #1 {
static_allocas:
  %2 = load i64* %0, align 8
  %3 = load i64* %1, align 8
  store i64 %3, i64* %0, align 8
  store i64 %2, i64* %1, align 8
  ret void
}
```

Another example:

```rust
#[inline(never)]
fn set<T>(x: &mut T, y: T) {
    *x = y;
}
```

Before, with `(int, int)` (align 1):

```llvm
define internal fastcc void @_ZN8set_282517_8fa972e3f9e451983_00E({ i64, i64 }* nocapture, { i64, i64 }* nocapture) #1 {
static_allocas:
  %2 = bitcast { i64, i64 }* %1 to i8*
  %3 = bitcast { i64, i64 }* %0 to i8*
  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 16, i32 1, i1 false)
  ret void
}
```

After, with `(int, int)` (align 8):

```llvm
define internal fastcc void @_ZN8set_282617_8fa972e3f9e451983_00E({ i64, i64 }* nocapture, { i64, i64 }* nocapture) #1 {
static_allocas:
  %2 = bitcast { i64, i64 }* %1 to i8*
  %3 = bitcast { i64, i64 }* %0 to i8*
  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 16, i32 8, i1 false)
  ret void
}
```