LDC sources structure refactoring

[Ivan Butygin via digitalmars-d-ldc]

Hi everyone,
I am working on my dynamic compilation PR 
(https://github.com/ldc-developers/ldc/pull/2293) and I need to 
link different from main compiler set of LLVM libraries to my jit 
runtime. But I can't do find_library(LLVM) in runtime cmake 
because there is already one in top level cmake. I know there are 
multiple ways to workaround this (and I will probably do this) 
but proper way to do this will be to move compiler sources to 
separate directory with own cmakelists and do find_library(LLVM). 
This will not be an easy task as it was explained in gitter :)
But we can move there incrementally and I created PR to move 
sources to separate dir with minimal changes in cmake:
https://github.com/ldc-developers/ldc/pull/2332

What do you think?