Only import _Concurrency implicitly on Darwin and Windows for now.

Author: DougGregor

lib/Frontend/Frontend.cpp

@@ -753,8 +753,18 @@ CompilerInstance::openModuleDoc(const InputFile &input) {
   return None;
 }
 
+/// Enable Swift concurrency on a per-target basis
+static bool shouldImportConcurrencyByDefault(const llvm::Triple &target) {
+  if (target.isOSDarwin())
+    return true;
+  if (target.isOSWindows())
+    return true;
+  return false;
+}
+
 bool CompilerInvocation::shouldImportSwiftConcurrency() const {
-  return !getLangOptions().DisableImplicitConcurrencyModuleImport &&
+  return shouldImportConcurrencyByDefault(getLangOptions().Target) &&
+      !getLangOptions().DisableImplicitConcurrencyModuleImport &&
       getFrontendOptions().InputMode !=
         FrontendOptions::ParseInputMode::SwiftModuleInterface;
 }