Stabilize ManuallyDrop::take

[CAD97]

Tracking issue: closes #55422
FCP merge: #55422 (comment)

Reclaims the doc improvements from closed #62198.

---

Stable version is a simple change if necessary.

Proposal: [relnotes] (this changes how to best take advantage of ManuallyDrop, esp. wrt. Drop::drop and finalize-by-value members)

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2020-01-09T18:21:57.4773862Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-01-09T18:21:57.4863339Z ##[command]git config gc.auto 0
2020-01-09T18:21:57.4954161Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-01-09T18:21:57.5028742Z ##[command]git config --get-all http.proxy
2020-01-09T18:21:57.5169928Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/68066/merge:refs/remotes/pull/68066/merge
---
2020-01-09T18:26:51.9814565Z     Checking panic_unwind v0.0.0 (/checkout/src/libpanic_unwind)
2020-01-09T18:26:56.9953112Z error: the feature `manually_drop_take` has been stable since 1.42.0 and no longer requires an attribute to enable
2020-01-09T18:26:56.9955545Z    --> src/libstd/lib.rs:278:12
2020-01-09T18:26:56.9956314Z     |
2020-01-09T18:26:56.9956947Z 278 | #![feature(manually_drop_take)]
2020-01-09T18:26:56.9958816Z     |
2020-01-09T18:26:56.9959516Z     = note: `-D stable-features` implied by `-D warnings`
2020-01-09T18:26:56.9959820Z 
2020-01-09T18:26:57.1484266Z error: aborting due to previous error
---
2020-01-09T18:26:57.1740185Z   local time: Thu Jan  9 18:26:57 UTC 2020
2020-01-09T18:26:57.4632250Z   network time: Thu, 09 Jan 2020 18:26:57 GMT
2020-01-09T18:26:57.4635094Z == end clock drift check ==
2020-01-09T18:26:58.7467296Z 
2020-01-09T18:26:58.7630996Z ##[error]Bash exited with code '1'.
2020-01-09T18:26:58.7657976Z ##[section]Starting: Checkout
2020-01-09T18:26:58.7659892Z ==============================================================================
2020-01-09T18:26:58.7659949Z Task         : Get sources
2020-01-09T18:26:58.7659999Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[Mark-Simulacrum]

Approving per FCP conclusion (with no further comments) on #55422; @bors r=Amanieu,Mark-Simulacrum -- I verified that the PR is still in good shape as well.

[bors]

📌 Commit 4e98966 has been approved by Amanieu,Mark-Simulacrum

[bors]

⌛ Testing commit 4e98966 with merge b5a3341...

[bors]

☀️ Test successful - checks-azure
Approved by: Amanieu,Mark-Simulacrum
Pushing b5a3341 to master...