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]

Address review comments:

• Simplify test
• Only perform the ClaimAll() if we actually link libc, so we get the warning back when combining -nostdlib/-nodefaultlibs with -pthread

[MatzeB]

This was accepted on the mailinglist and committed in r294065

lib/Driver/Tools.cpp

void darwin::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     if (getToolChain().getDriver().CCCIsCXX())
       getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
 
     // link_ssp spec is empty.
 
     // Let the tool chain choose which runtime library to link.
     getMachOToolChain().AddLinkRuntimeLibArgs(Args, CmdArgs);
+
+    // No need to do anything for pthreads. Claim argument to avoid warning.
+    Args.ClaimAllArgs(options::OPT_pthread);
+    Args.ClaimAllArgs(options::OPT_pthreads);
   }
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
     // endfile_spec is empty.
   }
 
   Args.AddAllArgs(CmdArgs, options::OPT_T_Group);
   Args.AddAllArgs(CmdArgs, options::OPT_F);

test/Driver/darwin-ld-pthread.c

+// RUN: %clang -Wunused-command-line-argument -pthread -target x86_64-apple-darwin -### /dev/null -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'