Auto merge of #143474 - jieyouxu:bootstrap-llvm-snapshot, r=Kobzol

Pretend in bootstrap snapshot tests that we always build in-tree LLVM

Otherwise, depending on whether CI LLVM is inhibited or if an externally-provided LLVM is used, bootstrap host LLVM build step could be missing in step snapshots.

Note that I'm not sure if this is the *right* solution (this might be *a* solution). I imagine we do want to control for the set of configuration that these snapshot tests are run, as much as possible.

r? `@Kobzol`

Author: bors