[Driver] Updated for Visual Studio 2017

Summary:
The patch updates the MSVC ToolChain for the changes made in Visual
Studio 2017[1].

Other notable changes:
  - Path handling code has been centralised to make potential future
    changes less painful.
  - A compiler error is emitted if the driver is unable to locate a
    usable MSVC toolchain. (Previously it'd fail with a cryptic error
    such as "link.exe is not executable")
  - Support for the new Setup Config Server API[2] has been added,
    albeit block commented out with a preprocessor conditional. This can
    probably be re-evaluated when the API is officially released (it's
    currently at the RC stage), but it's left in to make it easy for
    anyone familiar with the API to give it a go with Clang.

Patch by Hamza Sood.

[1] https://blogs.msdn.microsoft.com/vcblog/2016/10/07/compiler-tools-layout-in-visual-studio-15/
[2] https://blogs.msdn.microsoft.com/heaths/2016/09/15/changes-to-visual-studio-15-setup/

Reviewers: ruiu, hans, rnk

Reviewed By: rnk

Subscribers: awson, RKSimon, amccarth, cfe-commits

Differential Revision: https://reviews.llvm.org/D28365

llvm-svn: 293923