When enabled, send a notification before/after every "compilation",
which for now means `performSema`. This piggy-backs and modifies some
existing code that we had for "tracing" operations in sourcekitd that
unfortunately was untested. At least now some of the basic parts are
tested via the new notifications.
Part of rdar://38438512
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.