[clang-tools-extra] [clang-tidy] Add new check readability-trailing-comma (PR #173669)

[Baranov Victor via cfe-commits]

vbvictor wrote:

I agree that calculating lines between start/end of stmt/expr is not hard and we should do that way. 

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The check was initially started as a reflection of a coding guide I have at work: split init-list into multiple lines if number of elements is more than N and add a trailing comma.
But depending on a number of lines instead of number of elements is indeed seems more beneficial for wider audience. I'll address all comments next week. 


https://github.com/llvm/llvm-project/pull/173669