It was removed on recent versions of macOS and these entries break
sandboxing if they don't exist.
Aborted: while setting up the build environment: getting attributes of path '/System/Library/PrivateFrameworks/Ubiquity.framework/Versions/A/Ubiquity': No such file or directory
This also updates the bootstrap tool builder to LLVM 5, but not the ones
we actually use for bootstrap. I'll make that change in a subsequent commit
so as to provide traceable provenance of the bootstrap tools.
toPath has confusing semantics and is never necessary; it can always
either just be omitted or replaced by pre-concatenating `/.`. It has
been marked as "!!! obsolete?" for more than 10 years in a C++
comment, hopefully removing it will let us properly deprecate and,
eventually, remove it.
