This is required to correctly use the mock SDK when the SDK overlay is
built and tested separately. (Otherwise, the mock SDK might not get
used, because the overlay SDK options would expand from the
%-substitution, appear first on the command line, and shadow the mock
SDK in the search path).
Swift SVN r25185
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
