This refactors notification handling so that the XPC service and InProc
library pass in their notification handler callback. This fixes a
layering issue where the sourcekitd libraries could not be linked
dynamically due to the missing symbols.
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.
