[ms] [llvm-ml] Add placeholder for llvm-ml, based on llvm-mc

Summary:
As discussed on the mailing list, I plan to introduce an ml-compatible MASM assembler as part of providing more of the Windows build tools. This will be similar to llvm-mc, but with different command-line parameters.

This placeholder is purely a stripped-down version of llvm-mc; we'll eventually add support for the Microsoft-style command-line flags, and back it with a MASM parser.

Reviewers: rnk, thakis

Reviewed By: thakis

Subscribers: merge_guards_bot, mgorny, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72679