[PATCH] [Driver] Move description of toolchain specific include directories from the common driver code to the corresponding `MultilibSet` declarations

[Rafael Ávila de Espíndola]

There is a functionality change in that CSMipsMultilibs will not search InstallDir + "/../../../../sysroot/usr/include". Is that expected?

================
Comment at: lib/Driver/ToolChains.cpp:1838
@@ -1817,1 +1837,3 @@
+        return Dirs;
+      });
   }
----------------
This is identical to the first lambda, no? Can you factor it out?

http://reviews.llvm.org/D4720