Fix for compiling with clang <= 3.7 and g++6 headers.

[marsupial]

Summary

Make integers explicitly unsigned, so the tuple constructor will resolve properly when but with clang 3.6, 3.7 and gcc 6.1.1 libstdc++ headers.

[vsk]

LGTM.

Let me know if you'd like me to commit this for you.

[marsupial]

Would be nice...I have no commit access.
https://reviews.llvm.org/D22797 Is the LLVM portion, if they should land together

cfe/trunk/lib/Driver/Tools.cpp

if (Arg *A = Args.getLastArg(options::OPT_mdynamic_no_pic)) {
 
     // FIXME: Warn when this flag trumps some other PIC or PIE flag.
 
     // Only a forced PIC mode can cause the actual compile to have PIC defines
     // etc., no flags are sufficient. This behavior was selected to closely
     // match that of llvm-gcc and Apple GCC before that.
     PIC = ToolChain.isPICDefault() && ToolChain.isPICDefaultForced();
 
-    return std::make_tuple(llvm::Reloc::DynamicNoPIC, PIC ? 2 : 0, false);
+    return std::make_tuple(llvm::Reloc::DynamicNoPIC, PIC ? 2U : 0U, false);
   }
 
   if (PIC)
-    return std::make_tuple(llvm::Reloc::PIC_, IsPICLevelTwo ? 2 : 1, PIE);
+    return std::make_tuple(llvm::Reloc::PIC_, IsPICLevelTwo ? 2U : 1U, PIE);
 
-  return std::make_tuple(llvm::Reloc::Static, 0, false);
+  return std::make_tuple(llvm::Reloc::Static, 0U, false);
 }
 
 static const char *RelocationModelName(llvm::Reloc::Model Model) {
   switch (Model) {
   case llvm::Reloc::Static:
     return "static";
   case llvm::Reloc::PIC_:
     return "pic";