For a concrete type, members from its conforming protocols' extensions can be hard
to manually surface. In this commit, when printing Swift modules, we start to replicate these
extensions and synthesize them as if they are the concrete type's native extensions.
Credit to Doug for suggesting this practice.
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.
