Test for the specific proc_macro_span API expected by proc-macro2

[dtolnay]

This prevents the disruptive failure mode from #398 with projects using old pinned versions of crates against newer versions of Rust nightly compiler.

The anyhow and thiserror build script have been using this approach for over 4 years, and while there were issues to iron out with the build script logic, with build systems (Cargo, Nix, rustc bootstrap), IDEs (rust-analyzer, IntelliJ, RustRover), and other tools (cargo-expand, sccache), at this point the approach is reliable enough to deploy in proc-macro2.

• Redirect probe compile errors to /dev/null anyhow#91
• The no-op always-success RUSTC_WRAPPER breaks build scripts rust-lang/rust-analyzer#12973
• Do not unconditionally succeed RUSTC_WRAPPER when run by build scripts intellij-rust/intellij-rust#9235
• buildRustCrate: do not overlay OUT_PATH on src/ NixOS/nixpkgs#78706
• Include Cargo rustc configuration in probe anyhow#157
• Inform build scripts of rustc compiler context rust-lang/cargo#9601
• v1.0.52 update causing constant recompilation thiserror#270
• ./x.py test src/test/ui fails to build compiletest rust-lang/rust#100062
• build.rs checks for CARGO_RUSTC_WRAPPER which does not seem to be a thing anyhow#248
• propagate --target to rustc invocation anyhow#249
• "#![feature] may not be used on the stable release channel" anyhow#250
• Work around RA's RUSTC_WRAPPER poisoning all subsequent command-line builds anyhow#252
• 1.0.61 causes rebuilds after restarting VScode with rust-analyzer anyhow#261
• build probes confused by cargo always setting TARGET, even for build scripts of host dependencies rust-lang/cargo#11138
• don't run build probes in rustc bootstrap anyhow#320
• don't run build probes in rustc bootstrap thiserror#248
• Build caching issue: stage 0 fails in thiserror with "unresolved imports std::any::Demand, std::any::Provider" rust-lang/rust#114839
• Rerun build script on RUSTC_BOOTSTRAP change thiserror#269
• Set RUSTC_BOOTSTRAP only if needed cargo-expand#204
• Respect RUSTC environment variable if present cargo-expand#205
• Respect pre-existing RUSTC_BOOTSTRAP setting cargo-expand#206
• 1.0.53 interacts badly with sccache: feature may not be used on the stable release channel thiserror#278

The only known outstanding issue is RustRover spurious rebuilds when running unit tests through RustRover's test tool window, which has a fix in progress. https://youtrack.jetbrains.com/issue/RUST-13133