We should have the LLVM build which provides most of the tools that we need.
`strings` is amongst them. Add a new `llvm-strings` substitution from lit and
use that instead of `strings`. This helps with Windows which does not have a
`strings` tool.
Actually, the biggest win here seems to be not recording parameters,
which were taking up a ridiculous amount of space in the generated
swiftdoc. This change takes Swift.swiftdoc from 5MB to 3.5MB.