[clang] [clang-format] Fix repeated backslash insertion in macro line comments (PR #164300)

[Björn Schäpers via cfe-commits]

https://github.com/HazardyKnusperkeks commented:

You may also want to update the commit message, because line comments **can** be continued with `\`. https://godbolt.org/z/GP8YnoTqb

https://github.com/llvm/llvm-project/pull/164300