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

When performing multi-threaded IRGen, all of the link libraries used
for autolinking went (only) into the first object file. If the object
files were then placed in a static archive, the autolinking would only
kick in if clients that link the static library reference something in
the first object file. This causes link failures.

Make sure that we put the autolink information into each of the object
files we create when doing multi-threaded IRGen.

Fixes the rest of rdar://162400654.

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"}