[cxx-interop] Add -lc++ when C++ interop is enabled on Darwin.

Author: zoecarver

lib/IRGen/GenDecl.cpp

@@ -468,6 +468,10 @@ void IRGenModule::emitSourceFile(SourceFile &SF) {
 
   if (ObjCInterop)
     this->addLinkLibrary(LinkLibrary("objc", LibraryKind::Library));
+
+  // Automatically with libc++ when possible.
+  if (Context.LangOpts.EnableCXXInterop && Context.LangOpts.Target.isOSDarwin())
+    this->addLinkLibrary(LinkLibrary("c++", LibraryKind::Library));
 
   // FIXME: It'd be better to have the driver invocation or build system that
   // executes the linker introduce these compatibility libraries, since at

test/Interop/Cxx/stdlib/use-std-vector.swift

@@ -1,4 +1,4 @@
-// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-cxx-interop -lc++)
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-cxx-interop)
 //
 // REQUIRES: executable_test
 //