Rollup merge of #129473 - Urgau:fix-llvm-if-unchanged, r=onur-ozkan

use  `download-ci-llvm=true` in the default compiler config

rust-lang/rust@1ca2708 made it so that the `src/llvm-project` submodule has to be checkout for `download-ci-llvm = "if-unchanged"` to know if the submodule has been changed, but that is not required, if the submodule hasn't been checkout it cannot have been modified.

~~This PR restore the previous behavior by only updating the submodule if it has already been checkout.~~

This PR makes `download-ci-llvm = true` check if CI llvm is available and make it the default for the compiler profile, as to prevent unnecessarily checking out `src/llvm-project` with `"if-unchanged"`.

r? `````@onur-ozkan`````

Author: workingjubilee