Update lldb architecture docs

Summary:
Due to recent refactors, the descriptions of various modules were wildly
out of date. With this patch, I am not trying to legislate anything,
I am merely documenting the current state of affairs.

I am also deleting one copy of the architecture docs. AFAIK, this one is
not referenced from the web page.

Reviewers: zturner, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D34872

llvm-svn: 307072