Make getToolChain actually cache its result, as documented in Driver.h

[chandlerc]

This isn't necessary. The TC that we assign to is a reference to the pointer in the map. It should already be caching.

lib/Driver/Driver.cpp

default:
       else if (Target.isOSBinFormatELF())
         TC = new toolchains::Generic_ELF(*this, Target, Args);
       else if (Target.isOSBinFormatMachO())
         TC = new toolchains::MachO(*this, Target, Args);
       else
         TC = new toolchains::Generic_GCC(*this, Target, Args);
       break;
     }
+    ToolChains[Target.str()] = TC;
   }
   return *TC;
 }
 
 bool Driver::ShouldUseClangCompiler(const JobAction &JA) const {
   // Say "no" if there is not exactly one input of a type clang understands.
   if (JA.size() != 1 || !types::isAcceptedByClang((*JA.begin())->getType()))
     return false;