Add .clang-format file

[workingjubilee]

Is it possible to add a .clang-format file to configure the whole llvm-wrapper? The exists code does not fit any preset styles of clang-format. I have asked at discord but get no response about what code convention the cpp code follows.

Originally posted by @Lambdaris in #123437 (comment)

@Zalathar mentioned we have no such automated infra in place. We have many open issues for removing this C++ code by effectively upstreaming our wrappers to LLVM, but I agree, we could start by enforcing a style.

[dianqk]

I would like to add a check that follows to the LLVM style and run this check on CI.

I'm uncertain about adding a check locally, since most people won't be modifying the C++ code. It would require users to install clang-format on their local machines. Perhaps we could skip the check locally or only run it if clang-format is detected in the user's $PATH.

@rustbot claim

[Zalathar]

I wonder if it would be worthwhile to build clang-format as part of our LLVM build, and use that.

Obviously that doesn't help in cases where LLVM is downloaded (unless we also include it in the download), but maybe it's fine to skip the local check in those cases.

[workingjubilee]

As long as our C++ code doesn't introduce additional barriers to contributing to the Rust code.

[dianqk]

Currently, there are no clang-related builds here. Typically, we don't need to ensure that clang-format uses the same version of the code. I think that a local warning would be sufficient.

[durin42]

For Mercurial we found we had to be pretty particular about what version of clang-format we used in our test coverage, or some things would be fussy in some corner cases. I imagine that would be the case for us too, and it makes sense to me to take clang-format from the LLVM build so it's a consistent version...