Clang currently supports disabling color diagnostic output via -fno-color-diagnostics. However, there is a somewhat long-standing push to support use of an environment variable to override color output so that users can set up their terminal such that most color output is disabled (largely for accessibility reasons).
This patch adds support for NO_COLOR as that appears to be the more commonly supported feature, at least when comparing issues and pull requests:
https://github.com/search?q=NO_COLOR&type=issues (2.2k issues, 35k pull requests)
https://github.com/search?q=CLICOLOR&type=issues (1k issues, 3k pull requests)
It's also the more straightforward and thoroughly-specified of the two options. If NO_COLOR is present as an environment variable (regardless of value), color output is suppressed unless the command line specifies use of color output (command line takes precedence over the environment variable).