[PATCH] Improve Windows toolchain support for non-standard environments.

[Hans Wennborg]

lgtm

================
Comment at: lib/Driver/Tools.cpp:7825
@@ +7824,3 @@
+    llvm::sys::path::append(VSDir, Exe);
+    if (llvm::sys::fs::can_execute(VSDir.c_str()))
+      return VSDir.str();
----------------
nit: I'd just use VSDir.str(). Same for FilePath below.

http://reviews.llvm.org/D5845