swift234

Record whether a file contains a main entry point in the serialized module.

This isn't being used for much yet, but it will allow us to tell whether
something is an app target even at the module-merging stage, which is a
better way to tell if something is an app than whether it has a bridging
header.

We'll also need this if we ever take advantage of reusing "partial modules"
(serialized ASTs for other parts of the module that aren't affected by the
current source file) for compiling source files in incremental builds;
unfortunately that's unlikely given our dependency model.

Swift SVN r24531