Make the new behavior the default.

Author: David Ungar

include/swift/Option/Options.td

@@ -140,10 +140,9 @@ Flag<["-"], "enable-fine-grained-dependencies">, Flags<[FrontendOption, HelpHidd
 HelpText<"Experimental work-in-progress to be more selective about incremental recompilation">;
 
 
-def only_one_dependency_file :
-Flag<["-"], "only-one-dependency-file">, Flags<[DoesNotAffectIncrementalBuild]>,
- HelpText<"Experimental flag for faster thousand-source-file incremental builds">;
-
+def disable_only_one_dependency_file :
+Flag<["-"], "disable-only-one-dependency-file">, Flags<[DoesNotAffectIncrementalBuild]>,
+ HelpText<"Disables incremental build optimization that only produces one dependencies file">;
 
 def enable_source_range_dependencies :
 Flag<["-"], "enable-source-range-dependencies">, Flags<[]>,

lib/Driver/Driver.cpp

@@ -953,7 +953,7 @@ Driver::buildCompilation(const ToolChain &TC,
         createStatsReporter(ArgList.get(), Inputs, OI, DefaultTargetTriple);
 
     const bool OnlyOneDependencyFile =
-        ArgList->hasArg(options::OPT_only_one_dependency_file);
+        !ArgList->hasArg(options::OPT_disable_only_one_dependency_file);
     // relies on the new dependency graph
 
     const bool EnableFineGrainedDependencies =