Driver: Do not warn about unused -pthread when linking on darwin

[MatzeB]

Summary

While there is nothing to do at link time to get pthreads support on
darwin, specifying -pthread is fine and should not produce a warning
about unused arguments.

[MatzeB]

Add cfe-commits.

[MatzeB]

Abandoning in favor of https://reviews.llvm.org/D29479 because phabricator...

lib/Driver/Tools.cpp

if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     if (Arg *A = Args.getLastArg(options::OPT_fveclib)) {
       if (A->getValue() == StringRef("Accelerate")) {
         CmdArgs.push_back("-framework");
         CmdArgs.push_back("Accelerate");
       }
     }
   }
 
+  // No need to do anything for pthreads. Claim argument to avoid warning.
+  Args.ClaimAllArgs(options::OPT_pthread);
+
   const char *Exec = Args.MakeArgString(getToolChain().GetLinkerPath());
   std::unique_ptr<Command> Cmd =
       llvm::make_unique<Command>(JA, *this, Exec, CmdArgs, Inputs);
   Cmd->setInputFileList(std::move(InputFileList));
   C.addCommand(std::move(Cmd));
 }
 
 void darwin::Lipo::ConstructJob(Compilation &C, const JobAction &JA,

test/Driver/darwin-ld-pthread.c

+// RUN: %clang -target x86_64-apple-darwin -c -o %t.o %s
+// RUN: %clang -Wunused-command-line-argument -pthread -target x86_64-apple-darwin -### %t.o -o %t.bin 2>&1 | FileCheck %s
+
+// There is nothing to do at link time to get pthread support. But do not warn.
+// CHECK-NOT: argument unused during compilation: '-pthread'