Driver: Fix include and lib dirs when not using gcc under mingw

[martell]

Summary

When gcc is not installed we fail to set the correct include and lib directory.
In some cases base is set and we might want to use clang without gcc being installed

[yaron.keren]

This LGTM after modifying test/Driver/mingw.cpp to test for this, i.e. create a Inputs/no_gcc_tree/usr directory tree without the gcc dirs, point sysroot and check the output directories. It's mostly copy & paste from one of the existing cases.

[martell]

Updated to address yaron's comments.

Okay for me to merge now ?

[yaron.keren]

Yes, go ahead.

[martell]

Hi Hans,

Is this bugfix something that can be merged into 3.7 ?

Kind Regards
Martell

lib/Driver/MinGWToolChain.cpp

   Archs.emplace_back(getTriple().getArchName());
   Archs[0] += "-w64-mingw32";
   Archs.emplace_back("mingw32");
-  Arch = "unknown";
+  Arch = Archs[0].str();
   // lib: Arch Linux, Ubuntu, Windows
   // lib64: openSUSE Linux
   for (StringRef CandidateLib : {"lib", "lib64"}) {