Add .clang-tidy configuration file to provide LLVM-optimized defaults for clang-tidy.

[djasper]

I think this is a no-brainer.

[djasper]

The only question we might think about in the long(er) run is whether we really want ".clang-tidy" here but keep clang-tidy in clang-tools-extra.

[alexfh]

In D5236#7, @djasper wrote:

The only question we might think about in the long(er) run is whether we really want ".clang-tidy" here but keep clang-tidy in clang-tools-extra.

Yes, we should think about moving clang-tidy to the main tree. But I don't think the location of clang-tidy sources matters in this context. The file in question is a tool configuration file. We also have .arcconfig and .gitignore here, but nobody asks whether it's fine that the relevant tools are developed separately ;)

.clang-tidy

+Checks: '-*,llvm-*,misc-*'