[Matrix] Add draft specification for matrix support in Clang.

This patch documents the planned matrix support in Clang, based on the
draft specification discussed on cfe-dev in the 'Matrix Support in
Clang' thread.

Latest draft spec sent to cfe-dev: http://lists.llvm.org/pipermail/cfe-dev/2020-February/064742.html
Discussion thread January: http://lists.llvm.org/pipermail/cfe-dev/2020-January/064206.html
Discussion thread March: http://lists.llvm.org/pipermail/cfe-dev/2020-March/064834.html

Reviewers: rsmith, anemet, Bigcheese, dexonsmith, rjmccall

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D76612