Before this commit, replace-version-placeholder hardcoded the path
defining CURRENT_RUSTC_VERSION (to avoid replacing it). After a refactor
moved the file defining it without changing the hardcoded path, the tool
started replacing the constant itself with the version number.
To avoid this from happening in the future, this changes the definition
of the constant to avoid the tool from ever matching it.
- Switch from `walkdir` to `ignore`. This required various changes to
make `skip` thread-safe.
- Ignore `build` anywhere in the source tree, not just at the top-level.
We support this in bootstrap, we should support it in tidy too.
As a nice side benefit, this also makes tidy a bit faster.
Before:
```
; hyperfine -i '"/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"'
Benchmark 1: "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"
Time (mean ± σ): 1.080 s ± 0.008 s [User: 2.616 s, System: 3.243 s]
Range (min … max): 1.069 s … 1.099 s 10 runs
```
After:
```
; hyperfine '"/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"'
Benchmark 1: "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"
Time (mean ± σ): 705.0 ms ± 1.4 ms [User: 3179.1 ms, System: 1517.5 ms]
Range (min … max): 702.3 ms … 706.9 ms 10 runs
```
