LLVM llvm/clang 12 toolchain with runtimes build

First pass at evolving the build to produce a pure LLVM llvm/clang 12 toolchain including runtimes and dynamically linked executables.
First pass at constructing a complete toolchain from a two stage build with configured runtimes. Please understand that I am very new to debian package development and building llvm projects. I am not extremely familiar with many of the tools involved in these projects so just be warned that my build modifications might be quite naive in places. Otherwise, I look forward to any discussions, questions, suggestions or corrections.

Regarding the cmake patches, I'm not sure how to properly prepare quilt patches for the project yet, but I did verify that my patches were functionally valid and being applied for building.

P.S. I put this together in a bit of a hurry, but I wanted to get it up. I'm happy to adapt to any process required for the project.