Issue Description:

Trying to do anything at a breakpoint in lldb produces this error on mac
error: Error in auto-import:
failed to get module '<package name>' from AST context

This doesn't happen on Ubuntu 15.10 and lldb works as expected

Steps to reproduce:
$ git clone https://github.com/aciidb0mb3r/sample-swift-package.git
$ cd sample-swift-package
$ swift build
$ lldb .build/debug/SamplePackage
(lldb) breakpoint set -f main.swift -l 2
(lldb) r
(lldb) p foo

adding args += ["-g"] before args += ["-L/usr/local/lib"] in method writeLinkNode(target: ) at
https://github.com/apple/swift-package-manager/blob/master/Sources/dep/llbuild.swift resolves the issue

not sure if should it be added with conditional for OSX?