Introduce a mechanism for reusing Python modules out of tree.

Right now our Python code does not all share a common root.  Tests and
scripts both contain python code that cannot take advantage of reusability
since they are unrelated siblings of each other.

In particular, this presents a problem for wanting to use third party
packages from both sides, since it does not make sense to copy the module
into both places.

This patch solves this by introducing a script lldb_shared.py which is a
very lightweight script that just searches up the tree until it finds a
root, and then imports a module from there.  That module knows how to
find all of the shared code that LLDB uses, and adjusts sys.path
accordingly to make them all visible.

llvm-svn: 250858