Coding Standards: Document library layering requirements & header isolation.

(I suppose these two pieces could be separated - but seemed related
enough)

As discussed on llvm-dev, this documents the general expectation of how
library layering should be handled. There are a few existing cases where
these constraints are not met, but as with most style guide things -
this is forward looking and provides guidance when cleaning up existing
code, it doesn't immediately require that all previous code be cleaned
up to match. (see: naming conventions, etc)

Differential Revision: https://reviews.llvm.org/D42771

llvm-svn: 324004