VS Integration: install toolsets for x64 too (PR18738)

This does exactly the same thing as for Win32, except it passes -m64 to
the compiler and the files go in a different directory.

Differential Revision: http://llvm-reviews.chandlerc.com/D2749

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201269 91177308-0d34-0410-b5e6-96231b3b80d8