Windows ToolChain: add VS bin dir to PogramPaths

We have a lot of fancy logic to find Visual Studio, which is currently used
to set the system header include paths.

Use the same code to set the ProgramPaths, which is used for finding programs
such as link.exe. Previously, Clang would just search PATH for link.exe,
but now it should find it if it's able to find Visual Studio.

Differential Revision: http://llvm-reviews.chandlerc.com/D1417