Added perf test for deeply nested closures (#72408)

[VFLashM]

This is a benchmark for issue rust-lang/rust#72408 requested in PR rust-lang/rust#72412.

[jyn514]

Not sure I understand this error ... maybe @Mark-Simulacrum has ideas.

Running wg-grammar: Doc + [Full, IncrFull, IncrUnchanged, IncrPatched]
[2020-09-16T03:27:40Z DEBUG collector::execute] Benchmark iteration 1/1
[2020-09-16T03:27:40Z INFO  collector::execute] run_rustc with incremental=false, build_kind=Doc, run_kind=Some(Full), patch=None
file:///tmp/.tmp7QEot7#rust-grammar:0.1.0
[2020-09-16T03:27:40Z DEBUG collector::execute] "/home/runner/work/rustc-perf/rustc-perf/cache/f4e4485a052857e5dd32ea29ceb7b1a8223e83cc/bin/cargo" "rustdoc" "--manifest-path" "Cargo.toml" "-p" "file:///tmp/.tmp7QEot7#rust-grammar:0.1.0" "--" "--wrap-rustc-with" "perf-stat"
[2020-09-16T03:27:40Z INFO  cargo::core::compiler::fingerprint] fingerprint error for rust-grammar v0.1.0 (/tmp/.tmp7QEot7)/Doc { deps: false }/TargetInner { ..: lib_target("rust-grammar", ["lib"], "/tmp/.tmp7QEot7/src/lib.rs", Edition2015) }
[2020-09-16T03:27:40Z INFO  cargo::core::compiler::fingerprint]     err: profile configuration has changed
 Documenting rust-grammar v0.1.0 (/tmp/.tmp7QEot7)
<not supported>;;instructions:u;0;100.00;;
<not supported>;;cycles:u;0;100.00;;
987.42;msec;task-clock;987423442;100.00;0.998;CPUs utilized
987.42;msec;cpu-clock;987423442;100.00;0.998;CPUs utilized
41462;;faults;987423442;100.00;0.042;M/sec
300684;;max-rss;3;100.00
    Finished dev [unoptimized + debuginfo] target(s) in 1.02s
0.998780097;;wall-time;4;100.00
collection took 1071.715866596s with 1 failed benchmarks
collector error: 1 benchmarks failed
++ kill 10750
##[error]Process completed with exit code 1.

[VFLashM]

Huh. This error doesn't seem to be related to my change.

[Mark-Simulacrum]

Error is due to packed-simd being broken on nightly, and can be more easily seen on perf.rust-lang.org/status.

This looks good to me, though! I'm going to go ahead and merge.