When trying to add a new diagnostic category (e.g. #154618) I discovered
`clang-format` really wanted to reformat these files.
My initial attempt was just to suppress the reformatting with `//
clang-format (on|off)` directives but reviewers preferred just
reformatting the files so these two files have been completely
reformatted.
`clang-format` has been disabled for the enum that declares the
`DIAG_START_*` constants because its much less readable after
formatting.
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