Make file / directory completion work properly on Windows.

Description

Make file / directory completion work properly on Windows.

There were a couple of problems with this function on Windows. Different
separators and differences in how tilde expressions are resolved for
starters, but in addition there was no clear indication of what the
function's inputs or outputs were supposed to be, and there were no tests
to demonstrate its use.

To more easily paper over the differences between Windows paths,
non-Windows paths, and tilde expressions, I've ported this function to use
LLVM-based directory iteration (in fact, I would like to eliminate all of
LLDB's directory iteration code entirely since LLVM's is cleaner / more
efficient (i.e. it invokes fewer stat calls)). and llvm's portable path
manipulation library.

Since file and directory completion assumes you are referring to files and
directories on your local machine, it's safe to assume the path syntax
properties of the host in doing so, so LLVM's APIs are perfect for this.

I've also added a fairly robust set of unit tests. Since you can't really
predict what users will be on your machine, or what their home directories
will be, I added an interface called TildeExpressionResolver, and in the
unit test I've mocked up a fake implementation that acts like a unix
password database. This allows us to configure some fake users and home
directories in the test, so we can exercise all of those hard-to-test
codepaths that normally otherwise depend on the host.

Differential Revision: https://reviews.llvm.org/D30789