Support ? Kleene macro operator in 2015

Closes #56668.

See that issue for rationale and discussion.

Crater will be needed and then, if all goes well, FCP.

Support ? Kleene macro operator in 2015

Closes #56668.

See that issue for rationale and discussion.

Crater will be needed (done in #60932 (comment), zero regressions) and then, if all goes well, FCP (in #60932 (comment)).

Pkgsrc changes:
 * Add a patch to llvm to deal with const dli_saddr.
 * Adapt two other patches.
 * Cross-build currently fails, so i386, powerpc and sparc64 bootstrap
   kits for 1.37.0 are built natively.  Missing aarch64 hardware, so that's
   not available yet.
 * Bump bootstrap requirements to 1.36.0 except for armv7-unknown-netbsd-eabihf
   which I've not managed to cross-build.

Upstream changes:

Version 1.37.0 (2019-08-15)
==========================

Language
--------
- `#[must_use]` will now warn if the type is contained in a [tuple][61100],
  [`Box`][62228], or an [array][62235] and unused.
- [You can now use the `cfg` and `cfg_attr` attributes on
  generic parameters.][61547]
- [You can now use enum variants through type alias.][61682] e.g. You can
  write the following:
  ```rust
  type MyOption = Option<u8>;

  fn increment_or_zero(x: MyOption) -> u8 {
      match x {
          MyOption::Some(y) => y + 1,
          MyOption::None => 0,
      }
  }
  ```
- [You can now use `_` as an identifier for consts.][61347] e.g. You can write
  `const _: u32 = 5;`.
- [You can now use `#[repr(align(X)]` on enums.][61229]
- [The  `?`/_"Kleene"_ macro operator is now available in the
  2015 edition.][60932]

Compiler
--------
- [You can now enable Profile-Guided Optimization with the `-C profile-generate`
  and `-C profile-use` flags.][61268] For more information on how to use profile
  guided optimization, please refer to the [rustc book][rustc-book-pgo].
- [The `rust-lldb` wrapper script should now work again.][61827]

Libraries
---------
- [`mem::MaybeUninit<T>` is now ABI-compatible with `T`.][61802]

Stabilized APIs
---------------
- [`BufReader::buffer`]
- [`BufWriter::buffer`]
- [`Cell::from_mut`]
- [`Cell<[T]>::as_slice_of_cells`][`Cell<slice>::as_slice_of_cells`]
- [`DoubleEndedIterator::nth_back`]
- [`Option::xor`]
- [`Wrapping::reverse_bits`]
- [`i128::reverse_bits`]
- [`i16::reverse_bits`]
- [`i32::reverse_bits`]
- [`i64::reverse_bits`]
- [`i8::reverse_bits`]
- [`isize::reverse_bits`]
- [`slice::copy_within`]
- [`u128::reverse_bits`]
- [`u16::reverse_bits`]
- [`u32::reverse_bits`]
- [`u64::reverse_bits`]
- [`u8::reverse_bits`]
- [`usize::reverse_bits`]

Cargo
-----
- [`Cargo.lock` files are now included by default when publishing executable crates
  with executables.][cargo/7026]
- [You can now specify `default-run="foo"` in `[package]` to specify the
  default executable to use for `cargo run`.][cargo/7056]

Misc
----

Compatibility Notes
-------------------
- [Using `...` for inclusive range patterns will now warn by default.][61342]
  Please transition your code to using the `..=` syntax for inclusive
  ranges instead.
- [Using a trait object without the `dyn` will now warn by default.][61203]
  Please transition your code to use `dyn Trait` for trait objects instead.

[62228]: rust-lang/rust#62228
[62235]: rust-lang/rust#62235
[61802]: rust-lang/rust#61802
[61827]: rust-lang/rust#61827
[61547]: rust-lang/rust#61547
[61682]: rust-lang/rust#61682
[61268]: rust-lang/rust#61268
[61342]: rust-lang/rust#61342
[61347]: rust-lang/rust#61347
[61100]: rust-lang/rust#61100
[61203]: rust-lang/rust#61203
[61229]: rust-lang/rust#61229
[60932]: rust-lang/rust#60932
[cargo/7026]: rust-lang/cargo#7026
[cargo/7056]: rust-lang/cargo#7056
[`BufReader::buffer`]: https://doc.rust-lang.org/std/io/struct.BufReader.html#method.buffer
[`BufWriter::buffer`]: https://doc.rust-lang.org/std/io/struct.BufWriter.html#method.buffer
[`Cell::from_mut`]: https://doc.rust-lang.org/std/cell/struct.Cell.html#method.from_mut
[`Cell<slice>::as_slice_of_cells`]: https://doc.rust-lang.org/std/cell/struct.Cell.html#method.as_slice_of_cells
[`DoubleEndedIterator::nth_back`]: https://doc.rust-lang.org/std/iter/trait.DoubleEndedIterator.html#method.nth_back
[`Option::xor`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.xor
[`RefCell::try_borrow_unguarded`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html#method.try_borrow_unguarded
[`Wrapping::reverse_bits`]: https://doc.rust-lang.org/std/num/struct.Wrapping.html#method.reverse_bits
[`i128::reverse_bits`]: https://doc.rust-lang.org/std/primitive.i128.html#method.reverse_bits
[`i16::reverse_bits`]: https://doc.rust-lang.org/std/primitive.i16.html#method.reverse_bits
[`i32::reverse_bits`]: https://doc.rust-lang.org/std/primitive.i32.html#method.reverse_bits
[`i64::reverse_bits`]: https://doc.rust-lang.org/std/primitive.i64.html#method.reverse_bits
[`i8::reverse_bits`]: https://doc.rust-lang.org/std/primitive.i8.html#method.reverse_bits
[`isize::reverse_bits`]: https://doc.rust-lang.org/std/primitive.isize.html#method.reverse_bits
[`slice::copy_within`]: https://doc.rust-lang.org/std/primitive.slice.html#method.copy_within
[`u128::reverse_bits`]: https://doc.rust-lang.org/std/primitive.u128.html#method.reverse_bits
[`u16::reverse_bits`]: https://doc.rust-lang.org/std/primitive.u16.html#method.reverse_bits
[`u32::reverse_bits`]: https://doc.rust-lang.org/std/primitive.u32.html#method.reverse_bits
[`u64::reverse_bits`]: https://doc.rust-lang.org/std/primitive.u64.html#method.reverse_bits
[`u8::reverse_bits`]: https://doc.rust-lang.org/std/primitive.u8.html#method.reverse_bits
[`usize::reverse_bits`]: https://doc.rust-lang.org/std/primitive.usize.html#method.reverse_bits
[rustc-book-pgo]: https://doc.rust-lang.org/rustc/profile-guided-optimization.html