Introduce Go coding standards for LLVM.

Rather than define our own standards, we adopt a set of best practices that
are already in use by the Go community.

Differential Revision: http://reviews.llvm.org/D5761

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219646 91177308-0d34-0410-b5e6-96231b3b80d8