[Multi-threaded IRGen] Ensure that autolink info gets into each object file

lib/IRGen/IRGen.cpp

@@ -1740,9 +1740,7 @@ static void performParallelIRGeneration(IRGenDescriptor desc) {
 
     (void)layoutStringsEnabled(*IGM, /*diagnose*/ true);
 
-    // Only need to do this once.
-    if (!IGMcreated)
-      IGM->addLinkLibraries();
+    IGM->addLinkLibraries();
     IGMcreated = true;
   }
 

test/IRGen/autolink_multithread.swift

@@ -0,0 +1,35 @@
+// This test checks that we produce autolink entries into the expected places
+// when performing multi-threaded IR generation.
+
+// RUN: %empty-directory(%t/src)
+// RUN: split-file %s %t/src
+
+// RUN: %target-swift-frontend -emit-module -o %t -module-name empty -module-link-name empty %S/../Inputs/empty.swift
+
+// RUN: %target-swift-frontend -emit-ir %t/src/A.swift %t/src/B.swift -I %t -Fsystem %S/Inputs/Frameworks -o %t/A.ll -o %t/B.ll -num-threads 2 -O -g -module-name test
+// RUN: %FileCheck --check-prefix=CHECK-A %s <%t/A.ll
+// RUN: %FileCheck --check-prefix=CHECK-B %s <%t/B.ll
+
+// Linux uses a different autolinking mechanism, based on
+// swift-autolink-extract. This file tests the Darwin mechanism.
+// UNSUPPORTED: autolink-extract
+// REQUIRES: OS=macosx
+
+
+//--- A.swift
+import empty
+
+public func f() -> String { "hello" }
+
+// CHECK-A: !llvm.linker.options = !{
+// CHECK-A: !{{[0-9]+}} = !{!{{"-lempty"|"/DEFAULTLIB:empty.lib"}}}
+
+//--- B.swift
+import LinkFramework
+
+public func useLibrarySym() {
+  let _ = LinkFramework.IComeFromLinkFramework
+}
+
+// CHECK-B: !llvm.linker.options = !{
+// CHECK-B: !{{[0-9]+}} = !{!"-framework", !"LinkFramework"}