[lldb-dev] About lldbHost

[Zachary Turner via lldb-dev]

After https://reviews.llvm.org/D29964, we finally have a starting point at
which we can begin unravelling the cross-project cyclic dependencies in
LLDB.  lldb/Utility now is very similar in spirit to llvm/Support.

But llvmSupport goes one step further and includes what lldb would normally
put under Host.  I think this makes some sense.  Practically all parts of a
codebase have need of a single OS abstraction layer.  So, I think that a
lot of the functionality currently in lldbHost is in fact needed by the
rest of LLDB.

So, I wonder if it makes sense to follow the path that LLVM has taken, and
start moving some of this code from Host down to Utility.  Doing so would
allow us to break one of the biggest links in the dependency cycle in the
entire codebase, which is that Host depends on everything, and everything
depends on Host.

Of course, it can't just be a straight move.  Some things in Host interact
with Target, with CommandInterpreter, and with many other things.  And
stuff going into Utility can't take a dependency.

So there will be some splitting, some moving, some refactoring, etc.  But
to me tackling Host seems like the logical next step, in large part because
Host is where FileSpec is, and it's going to be hard to break any
dependencies without first addressing FileSpec.

The way LLVM handles cross-platform differences in Support is that you
include a single header and it does conditional includes into a platform
specific subdirectory for the parts that differ.

I'm thinking to follow the same pattern here in lldb/Utility, and begin
looking for ways to get pieces of Host down into Utility this way, until
ultimately I can get FileSpec down there.

Thoughts?
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