Fix empty -L Path on OSX hosts

[martell]

Summary

Not quite sure if this is the proper way to fix this.
This is currently my work around when working from an OSX host though.

I think I need some feedback from either rnk or yaron on how to fix this.
I assume the path is supposed to be "." and not an empty string as it is on mac.
I can't reproduce this issue on linux or windows also.

[yaron.keren]

I have never used OSX, Try to add one of the Apple clang developers as reviewers, they know more than me about OSX.

My guess there should not be empty paths in in TC.getFilePaths(). That patch seems only to sidestep the issue which will probably surface in other places. Can you debug why this empty path is created and fix that?

[rnk]

This looks fine, just make sure the indentation is right. Phab seems confused.

[dblaikie]

Is this testable?

[martell]

Is this testable?

Seems to me the only way to add this as a test would be to add bad code to clang to purposely pass an empty LibPath argument.
This is probably something we do not want to do.

This is probably why Reid never asked me for one in the review.

cfe/trunk/lib/Driver/ToolChain.cpp

case ToolChain::CST_Libstdcxx:
     CmdArgs.push_back("-lstdc++");
     break;
   }
 }
 
 void ToolChain::AddFilePathLibArgs(const ArgList &Args,
                                    ArgStringList &CmdArgs) const {
   for (const auto &LibPath : getFilePaths())
+    if(LibPath.length() > 0)
       CmdArgs.push_back(Args.MakeArgString(StringRef("-L") + LibPath));
 }
 
 void ToolChain::AddCCKextLibArgs(const ArgList &Args,
                                  ArgStringList &CmdArgs) const {
   CmdArgs.push_back("-lcc_kext");
 }
 
 bool ToolChain::AddFastMathRuntimeIfAvailable(const ArgList &Args,