Add dependency on clang-headers when building LLDB.framework using CMake

Summary:
The LLDB.framework generated when building with CMake + Ninja/Make is
completely missing the clang headers. Although the code to copy them exists, we
don't even generate them unless we're building LLDB standalone.

Reviewers: clayborg, labath, sas

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D47612

llvm-svn: 333777