[llvm] [cmake] Enable color diagnostics (PR #79419)

[via llvm-commits]

cor3ntin wrote:

On my system (linux, ninja), I do get colors in the current and the doc of cmake seems to imply that the default is to let the compiler does what it does by default.

But i see tons of ninja-related bugs pertaining to colors, and I am not sure they fixed it recently or not.
So i can't tell whether this change is necessary (and how it impacts ci)

https://github.com/llvm/llvm-project/pull/79419