syntax: Parse GT tokens from `>=` and `>>=`

The parser already has special logic for parsing `>` tokens from `>>`, and this
commit extends the logic to the acquiring a `>` from the `>=` and `>>=` tokens
as well.

Closes #15043
auto merge of #15056 : alexcrichton/rust/issue-15043, r=kballard

The parser already has special logic for parsing `>` tokens from `>>`, and this
commit extends the logic to the acquiring a `>` from the `>=` and `>>=` tokens
as well.

Closes #15043
librustc: Put `#[unsafe_destructor]` behind a feature gate.

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]
auto merge of #14988 : pcwalton/rust/unsafe-destructor-feature-gate, r=alexcrichton

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]

r? @alexcrichton
librustc: Feature gate lang items and intrinsics.

If you define lang items in your crate, add `#[feature(lang_items)]`.

If you define intrinsics (`extern "rust-intrinsic"`), add
`#[feature(intrinsics)]`.

Closes #12858.

[breaking-change]