[lldb][CMake] Avoid 'Autogenerate scheme' dialogs in Xcode projects

Summary:
Supported in CMake 3.9 and higher: https://cmake.org/cmake/help/v3.9/variable/CMAKE_XCODE_GENERATE_SCHEME.html
Older versions will just report it as unused in the end of the configuration process.

Reviewers: jingham, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65509

llvm-svn: 367538