Build a flat LLDB.framework for embedded Darwin targets

This patch configures LLDB.framework to build as a flat unversioned
framework on non-macOS Darwin targets, which have never supported the
macOS framework layout.

This patch also renames the 'IOS' cmake variable to 'APPLE_EMBEDDED' to
reflect the fact that lldb is built for several different kinds of embedded
Darwin targets, not just iOS.

Differential Revision: https://reviews.llvm.org/D85770