Allow the use of declarations whose names start with $ in all
modes. However, normal code cannot define new entities with names that
start with $: only the implementation can do that, e.g., for property
delegates.
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.
