[CMake] Fix RPATH handling for LLDB.framework

Summary:
Generator expressions are not supported in the `BUILD_RPATH` target property.
`BUILD_RPATH` is only supported in 3.8+ https://cliutils.gitlab.io/modern-cmake/chapters/intro/newcmake.html
`LLDB_FRAMEWORK_INSTALL_DIR` should not overwrite, but rather add an install RPATH (and it should be the first)

Reviewers: xiaobai, lanza

Reviewed By: xiaobai

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D57989

llvm-svn: 354037