Adds a new flag "-experimental-skip-all-function-bodies" that skips
typechecking and SIL generation for all function bodies (where
possible).
`didSet` functions are still typechecked and have SIL generated as their
body is checked for the `oldValue` parameter, but are not serialized.
Parsing will generally be skipped as well, but this isn't necessarily
the case since other flags (eg. "-verify-syntax-tree") may force delayed
parsing off.
This may help us reproduce a failing build when all we have is a build
log, and will become much more important in batch mode when we
/really/ need to know what ended up in what batch.
For now, this doesn't include /output/ filelists, because David's
about to mess with that code anyway to make things better around
supplementary outputs in batch mode. There is one weirdness there,
though, which is that ArgsToFrontendInputsConverter peeks at the
outputs to see whether we're doing single-threaded or multi-threaded
WMO.
