Clang and Swift decls likely have many-to-one mappings; for instance,
a RecordDecl and a TypedefDecl in Clang are both imported as the same Swift
NominalTypeDecl. This commit reduces duplication when we print Clang decls in
Swift. rdar://23275959
At some point I want to propose a revised model for exports, but for now
just mark that support for '@exported' is still experimental and subject
to change. (Thanks, Max.)
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.
