Add llvm-go tool.

This tool lets us build LLVM components within the tree by setting up a
$GOPATH that resembles a tree fetched in the normal way with "go get".

It is intended that components such as the Go frontend will be built in-tree
using this tool.

Differential Revision: http://reviews.llvm.org/D5902

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220462 91177308-0d34-0410-b5e6-96231b3b80d8