... instead of an array of compiler arguments. This is good enough
for seeing what's going on, and it saves significant time for long
argument strings, because it doesn't create and destroy so many
xpc strings, and more of the string copying that happens is on a large
contiguous string instead of many small strings.
rdar://39538847
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