[CMake] Fix RPATH handling for LLDB.framework
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
Differential Revision: https://reviews.llvm.org/D57989