Replace `using namespace lldb` with explicit `using lldb::<name>`
imports.

The object change that commit b73e042 ("src,test: support node.js >= 8")
from April addressed has been reverted again in 8.1.0.  Update the test.

Refs: nodejs/node#13374

Previously, `v8 bt` would exclude frames that didn't map to a C++ symbol
or a JS stack frame.  llnode does not currently know how to identify the
stack frames of V8 builtins so those were omitted as well.

This commit makes those stack frames visible and introduces a heuristic
(in lldb >= 3.9) where frames whose PC is inside a WX memory segment are
assumed to belong to V8 builtins.

Fixes: #99