[CMake] Only support LLDB_BUILD_FRAMEWORK on CMake 3.7 and later

CMake's framework target generation was unable to generate POST_BUILD steps (see: https://gitlab.kitware.com/cmake/cmake/issues/16363).

It turns out working around this is really not reasonable. The more reasonable solution to me is just to not support LLDB.framework unless you are on CMake 3.7 or newer.

Since CMake 3.7.1 is released that's how I'm going to handle this.

llvm-svn: 289841