Auto merge of #37911 - liigo:rustdoc-playground, r=alexcrichton

rustdoc: get back missing crate-name when --playground-url is used

follow up PR #37763
r? @alexcrichton (since you r+ed to #37763 )

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Edit: When `#![doc(html_playground_url="")]` is used, the current crate name is saved to `PLAYGROUND`, so rustdoc may generate `extern crate NAME;` into code snips automatically. But when `--playground-url` was introduced in PR #37763, I forgot saving crate name to `PLAYGROUND`. This PR fix that.

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Update:
- add test
- unstable `--playground-url`

src/test/rustdoc/playground-arg.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: --playground-url=https://example.com/ -Z unstable-options
+// ignore-tidy-linelength
+
+#![crate_name = "foo"]
+
+//! ```
+//! use foo::dummy;
+//! dummy();
+//! ```
+
+pub fn dummy() {}
+
+// ensure that `extern crate foo;` was inserted into code snips automatically:
+// @matches foo/index.html '//a[@class="test-arrow"][@href="https://example.com/?code=extern%20crate%20foo%3B%0Afn%20main()%20%7B%0Ause%20foo%3A%3Adummy%3B%0Adummy()%3B%0A%7D"]' "Run"