llvm-bcanalyzer does *not* like it when there are multiple block info metadata blocks in the same bitstream file. This patch will skip the emission of that and just jump straight to the metadata block when we're not reading a "standalone" incremental dependency file. While I'm here, also add the right block abbreviation info so we can get a decent dump from llvm-bcanalyzer
Take advantage of the binary swiftdeps serialization utliities built during #32131. Add a new optional information block to swiftdeps files. For now, don't actually serialize swiftdeps information.
Frontends will use this information to determine whether to write incremental dependencies across modules into their swiftdeps files. We will then teach the driver to deserialize the data from this section and integrate it into its incremental decision making.
