[clang] [clang][Diagnostics] Make 'note' color CYAN (PR #66997)

[Christopher Di Bella via cfe-commits]

cjdb wrote:

> This is a one-liner that fixes the problem on terminals with a dark background and the solution also works on light backgrounds. So, basically all of them.

Before approving this change, I want confirmation that it doesn't adversely impact popular themes. The Tomorrow Night Blue theme that's in my Visual Studio Code client (not my theme, but one of the defaults, I think) does need to change the hue of its blue text.

If we can't detect the background, then I think it's extra important that we test this on as many themes as possible, and with colourblind filters as well.

https://github.com/llvm/llvm-project/pull/66997