Use the correct edition when syntax highlighting doctests

Previously it would unconditionally use edition 2015, which was
incorrect.

src/test/rustdoc-ui/doctest-edition.rs

@@ -0,0 +1,16 @@
+// edition:2021
+
+#![deny(rustdoc::invalid_rust_codeblocks)]
+//~^ NOTE lint level is defined here
+
+// By default, rustdoc should use the edition of the crate.
+//! ```
+//! foo'b'
+//! ```
+//~^^^ ERROR could not parse
+//~| NOTE prefix `foo` is unknown
+
+// Rustdoc should respect `edition2018` when highlighting syntax.
+//! ```edition2018
+//! foo'b'
+//! ```

src/test/rustdoc-ui/doctest-edition.stderr

@@ -0,0 +1,22 @@
+error: could not parse code block as Rust code
+  --> $DIR/doctest-edition.rs:7:5
+   |
+LL |   //! ```
+   |  _____^
+LL | | //! foo'b'
+LL | | //! ```
+   | |_______^
+   |
+note: the lint level is defined here
+  --> $DIR/doctest-edition.rs:3:9
+   |
+LL | #![deny(rustdoc::invalid_rust_codeblocks)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: error from rustc: prefix `foo` is unknown
+help: mark blocks that do not contain Rust code as text
+   |
+LL | //! ```text
+   |        ++++
+
+error: aborting due to previous error
+