Create empty shell of llvm-mt.

Summary:
This is the first patch towards creating the llvm-mt tool for merging
Windows manifests.  This is a reimplementation of mt.exe.

Reviewers: zturner, ruiu, rnk

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D35333

llvm-svn: 308224