[Dependency Scanning] Enable parallel dependency scanning by-default

Author: artemcm

include/swift/Frontend/FrontendOptions.h

@@ -358,7 +358,7 @@ class FrontendOptions {
 
   /// Whether the dependency scanner invocation should resolve imports
   /// to filesystem modules in parallel.
-  bool ParallelDependencyScan = false;
+  bool ParallelDependencyScan = true;
 
   /// When performing an incremental build, ensure that cross-module incremental
   /// build metadata is available in any swift modules emitted by this frontend

include/swift/Option/FrontendOptions.td

@@ -260,6 +260,8 @@ def dependency_scan_cache_remarks : Flag<["-"], "Rdependency-scan-cache">,
 
 def parallel_scan : Flag<["-"], "parallel-scan">,
    HelpText<"Perform dependency scanning in-parallel.">;
+def no_parallel_scan : Flag<["-"], "-no-parallel-scan">,
+   HelpText<"Perform dependency scanning in a single-threaded fashion.">;
 
 def enable_copy_propagation : Flag<["-"], "enable-copy-propagation">,
   HelpText<"Run SIL copy propagation with lexical lifetimes to shorten object "

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -142,7 +142,9 @@ bool ArgsToFrontendOptionsConverter::convert(
   Opts.SerializeDependencyScannerCache |= Args.hasArg(OPT_serialize_dependency_scan_cache);
   Opts.ReuseDependencyScannerCache |= Args.hasArg(OPT_reuse_dependency_scan_cache);
   Opts.EmitDependencyScannerCacheRemarks |= Args.hasArg(OPT_dependency_scan_cache_remarks);
-  Opts.ParallelDependencyScan |= Args.hasArg(OPT_parallel_scan);
+  Opts.ParallelDependencyScan = Args.hasArg(OPT_parallel_scan,
+                                            OPT_no_parallel_scan,
+                                            true);
   if (const Arg *A = Args.getLastArg(OPT_dependency_scan_cache_path)) {
     Opts.SerializedDependencyScannerCachePath = A->getValue();
   }