TargetLibraryInfo: Assume no libcalls in the default constructor (#92400)

The only tricky point here is PlaceSafepoints has an awful hack where
it's creating a legacy PassManager inside it's runImpl, which was not
propagating the incoming TLI. This means there's an implicit bug fix,
where PlaceSafepoints would have been treating too many calls as
builtins.

I'm trying to delete the default constructor altogether, but this seems
to be more difficult.