rust: proc-macro2: add SPDX License Identifiers

Originally, when the Rust upstream `alloc` standard library crate was
vendored in commit 057b8d257107 ("rust: adapt `alloc` crate to the
kernel"), the SPDX License Identifiers were added to every file so that
the license on those was clear.

Thus do the same for the `proc-macro2` crate.

This makes `scripts/spdxcheck.py` pass.

Reviewed-by: Gary Guo <[e-mail]>
Tested-by: Gary Guo <[e-mail]>
Tested-by: Jesung Yang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Miguel Ojeda <[e-mail]>

rust/proc-macro2/detection.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use core::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Once;
 

rust/proc-macro2/extra.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! Items which do not have a correspondence to any API in the proc_macro crate,
 //! but are necessary to include in proc-macro2.
 

rust/proc-macro2/fallback.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 #[cfg(wrap_proc_macro)]
 use crate::imp;
 #[cfg(span_locations)]

rust/proc-macro2/lib.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! [![github]](https://github.com/dtolnay/proc-macro2) [![crates-io]](https://crates.io/crates/proc-macro2) [![docs-rs]](crate)
 //!
 //! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github

rust/proc-macro2/location.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use core::cmp::Ordering;
 
 /// A line-column pair representing the start or end of a `Span`.

rust/proc-macro2/marker.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use alloc::rc::Rc;
 use core::marker::PhantomData;
 use core::panic::{RefUnwindSafe, UnwindSafe};

rust/proc-macro2/parse.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use crate::fallback::{
     self, is_ident_continue, is_ident_start, Group, Ident, LexError, Literal, Span, TokenStream,
     TokenStreamBuilder,

rust/proc-macro2/probe.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 #![allow(dead_code)]
 
 #[cfg(proc_macro_span)]

rust/proc-macro2/probe/proc_macro_span.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 // This code exercises the surface area that we expect of Span's unstable API.
 // If the current toolchain is able to compile it, then proc-macro2 is able to
 // offer these APIs too.

rust/proc-macro2/probe/proc_macro_span_file.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 // The subset of Span's API stabilized in Rust 1.88.
 
 extern crate proc_macro;

rust/proc-macro2/probe/proc_macro_span_location.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 // The subset of Span's API stabilized in Rust 1.88.
 
 extern crate proc_macro;

rust/proc-macro2/rcvec.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use alloc::rc::Rc;
 use alloc::vec;
 use core::mem;

rust/proc-macro2/wrapper.rs

···
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use crate::detection::inside_proc_macro;
 use crate::fallback::{self, FromStr2 as _};
 #[cfg(span_locations)]