Enable ExperientalPrivateIntransitiveDependencies By Default

"Private Intransitive Dependencies" differ from the status quo by no
longer requiring the concept of a "cascading dependency edge". This is
because the request evaluator automatically tracks, records, and replays
the names looked up while a given file is being processed by the
frontend. To remove transitivity from the swiftdeps files, each
primary file processed by the Swift frontend is charged for *all* name
lookups that occur while it is being processed.  Further, because of
the replay step, lookups hidden behind cached requests are now entirely
visible to the dependency tracking code.

The net result is that all formerly implicit transitivity in the
dependency graph has been made completely explicit and direct. This
establishes a tighter overall dependency structure for each individual
file, and results in a remarkable decrease in the amount of
files that are rebuilt for any particular change.

This feature can be disabled via
-disable-experimental-private-intransitive-dependencies, which will
cause a return to the cascading status quo.

Author: CodaFi

include/swift/Basic/LangOptions.h

@@ -367,7 +367,7 @@ namespace swift {
 
     /// Whether to enable a more aggressive mode of incremental dependency
     /// gathering that never captures cascading edges.
-    bool EnableExperientalPrivateIntransitiveDependencies = false;
+    bool EnableExperientalPrivateIntransitiveDependencies = true;
 
     /// Enable verification when every SubstitutionMap is constructed.
     bool VerifyAllSubstitutionMaps = false;