Implement -use-ld= switch in Swift

Author: tienex

include/swift/Option/Options.td

@@ -254,6 +254,10 @@ def no_link_objc_runtime : Flag<["-"], "no-link-objc-runtime">,
   Flags<[HelpHidden, DoesNotAffectIncrementalBuild]>,
   HelpText<"Don't link in additions to the Objective-C runtime">;
 
+def use_ld : Joined<["-"], "use-ld=">,
+  Flags<[DoesNotAffectIncrementalBuild]>,
+  HelpText<"Specifies the linker to be used">;
+
 def Xlinker : Separate<["-"], "Xlinker">,
   Flags<[DoesNotAffectIncrementalBuild]>,
   HelpText<"Specifies an option which should be passed to the linker">;

lib/Driver/ToolChains.cpp

@@ -1130,6 +1130,17 @@ toolchains::GenericUnix::constructInvocation(const LinkJobAction &job,
     Arguments.push_back(context.Args.MakeArgString(context.OI.SDKPath));
   }
 
+  // Select the linker to use
+  llvm::SmallString<128> Linker;
+
+  if (const Arg *A = context.Args.getLastArg(options::OPT_use_ld)) {
+    Linker = A->getValue();
+  }
+
+  if (!Linker.empty()) {
+    Arguments.push_back(context.Args.MakeArgString("-fuse-ld=" + Linker));
+  }
+
   // Add the runtime library link path, which is platform-specific and found
   // relative to the compiler.
   // FIXME: Duplicated from CompilerInvocation, but in theory the runtime