Rebuilding LLVM after a submodule update is too lazy

[Zalathar]

There's a pattern I've been noticing when building the compiler after a recent LLVM submodule update.

If I do git checkout master && git pull (which happens to include an LLVM submodule update), and then do x build library, the build system will make sure that the LLVM submodule is properly fetched, but it won't actually rebuild LLVM. Instead the build command exits successfully, presumably using the old LLVM artifacts.

Then if I do x build library a second time, the build system notices that LLVM is stale, and actually rebuilds it.

@rustbot label +T-bootstrap