Expose an ImportNonPublicDependencies LangOpt for LLDB (NFC).
This patch allows controlling the automatic import of private dependencies
separately from the DebuggerSupport option, which currently also triggers this
behavior. With explicit modules + precise compiler invocations LLDB is moving
towards no longer needing this behavior.
rdar://133088201
(cherry picked from commit a1ba7159e37638a1de9e12faababc7d791c83a85)
