[CMake] Populate LLDB.framework's headers directory

Summary:
This patch adds support for installing public headers in LLDB.framework, and symlinking the headers into the build directory.

While writing the patch I discovered a bug in CMake that prevents applying POST_BUILD steps to framework targets (https://gitlab.kitware.com/cmake/cmake/issues/16363).

I've implemented the support using POST_BUILD steps wrapped under a CMake version check with a TODO so that we can track the fix.

Reviewers: tfiala, zturner, spyffe

Subscribers: lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D25570

llvm-svn: 284250