[ExplicitModuleBuild] Don't leak chained bridging header in objc header
When generating an objc header from the swift module when a bridging
header is used, make sure to use the original bridging header, not
the chained bridging header. This also avoids incorrectly generated a
header include when no actual bridging header is used, just a chained
bridging header that is coming from a dependency.
rdar://148446465