Introduce install-lldb-framework target

Summary:
Previously, I thought that install-liblldb would fail because CMake had
a bug related to installing frameworks. In actuality, I misunderstood the
semantics of `add_custom_target`: the DEPENDS option refers to specific files,
not targets. Therefore `install-liblldb` should rely on the actual liblldb
getting generated rather than the target.

This means that the previous patch I committed (to stop relying on CMake's
framework support) is no longer needed and has been reverted. Using CMake's
framework support greatly simplifies the implementation.

`install-lldb-framework` (and the stripped variant) is as simple as
depending on `install-liblldb` because CMake knows that liblldb was built as a
framework and will install the whole framework for you. The stripped variant
will depend on the stripped variants of individual tools only to ensure they
actually are stripped as well.

Reviewers: labath, sas

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D50038

llvm-svn: 338594