Auto merge of #34563 - alexcrichton:robust-mk, r=brson

mk: Don't consider LLVM done until it's done

Currently if an LLVM build is interrupted *after* it creates the llvm-config
binary but before it's done it puts us in an inconsistent state where we think
LLVM is compiled but it's not actually. This tweaks our logic to only consider
LLVM done building once it's actually done building.

This should hopefully alleviate problems on the bots where if we interrupt at
the wrong time it doesn't corrupt the build directory.