[tools] Parallelize tools/clang_format_new.py (#34750)
Summary:
**Summary**
This commit parallelizes the invocation of `clang-format` on all files
in `tools/clang_format_new.py` using `asyncio`.
**Testing**
Ran and timed the script.
*Before*
```
$ time ./tools/clang_format_new.py --diff
...
real 0m7.615s
user 0m6.012s
sys 0m1.634s
```
*After*
```
$ time ./tools/clang_format_new.py --diff
...
Some files not formatted correctly
real 0m2.156s
user 0m8.488s
sys 0m3.201s
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34750
Differential Revision: D20523133
Pulled By: SplitInfinity
fbshipit-source-id: 509741a0b4fcfcdcd7c5a45654e3453b4874d256
