Auto merge of #76864 - est31:downloaded_llvm_no_clone_sources, r=Mark-Simulacrum
Auto merge of #76864 - est31:downloaded_llvm_no_clone_sources, r=Mark-Simulacrum

Don't download/sync llvm-project submodule if download-ci-llvm is set

llvm-project takes > 1GB storage space and a long time to download.
It's better to not download it unless needed.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust