Cursor info requires access to the underlying AST, which is not
thread-safe. This manifest as crashes when performing concurrent
cursor-info requests on the same generated interface. We already
prevented concurrent cursor-infos on regular Swift files by using the
ASTManager, but generated interfaces use the InterfaceGenContext which
may use either an ASTUnit or its own internal CompilerInstance.
rdar://problem/27311624
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.
