Unrolled build for rust-lang#126779

Rollup merge of rust-lang#126779 - Zalathar:rust-dev, r=Kobzol

Try to clarify the confusingly-named `RustDev` and `RustcDev` steps

When trying to track down how the downloaded LLVM gets built and bundled, I was greatly confused by these step names.

I wasn't sure whether I could just rename them (since they correspond to the filename of the resulting tarball), but I at least wanted to leave behind some signposts to reduce confusion.

src/bootstrap/src/core/build_steps/dist.rs

@@ -665,6 +665,10 @@ impl Step for Std {
     }
 }
 
+/// Tarball containing the compiler that gets downloaded and used by
+/// `rust.download-rustc`.
+///
+/// (Don't confuse this with [`RustDev`], without the `c`!)
 #[derive(Debug, PartialOrd, Ord, Clone, Hash, PartialEq, Eq)]
 pub struct RustcDev {
     pub compiler: Compiler,
@@ -2225,6 +2229,11 @@ impl Step for LlvmBitcodeLinker {
 /// Tarball intended for internal consumption to ease rustc/std development.
 ///
 /// Should not be considered stable by end users.
+///
+/// In practice, this is the tarball that gets downloaded and used by
+/// `llvm.download-ci-llvm`.
+///
+/// (Don't confuse this with [`RustcDev`], with a `c`!)
 #[derive(Clone, Debug, Eq, Hash, PartialEq)]
 pub struct RustDev {
     pub target: TargetSelection,