Fix swiftpm build

Author: aschwaighofer

lib/Driver/DarwinToolChains.cpp

@@ -427,6 +427,25 @@ toolchains::Darwin::constructInvocation(const LinkJobAction &job,
     }
   }
 
+  Optional<llvm::VersionTuple> runtimeDynamicReplacementCompatibilityVersion;
+  if (job.getKind() == LinkKind::Executable) {
+    runtimeDynamicReplacementCompatibilityVersion
+                         = getSwiftRuntimeCompatibilityVersionForTarget(Triple);
+    if (*runtimeDynamicReplacementCompatibilityVersion <=
+        llvm::VersionTuple(5, 0)) {
+      // Swift 5.0 dynamic replacement compatibility library.
+      SmallString<128> BackDeployLib;
+      BackDeployLib.append(RuntimeLibPath);
+      llvm::sys::path::append(BackDeployLib,
+                              "libswiftCompatibilityDynamicReplacements.a");
+
+      if (llvm::sys::fs::exists(BackDeployLib)) {
+        Arguments.push_back("-force_load");
+        Arguments.push_back(context.Args.MakeArgString(BackDeployLib));
+      }
+    }
+  }
+
   // Link the standard library.
   Arguments.push_back("-L");
   if (context.Args.hasFlag(options::OPT_static_stdlib,