[lldb][CMake] Generating Xcode projects

Summary:
Print a warning if the wrong cache script is used when generating a Xcode project, because it's too easy to confuse with Apple-lldb-macOS.cmake

```
  When building with Xcode, we recommend using the corresponding cache
  script.  If this was a mistake, clean your build directory and re-run
  CMake with:

    -C /path/to/llvm-project/lldb/cmake/caches/Apple-lldb-Xcode.cmake

  See: https://lldb.llvm.org/resources/build.html#cmakegeneratedxcodeproject
```

Also set the generator inside the cache script.

Reviewers: JDevlieghere, jingham, clayborg

Reviewed By: JDevlieghere

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65797

llvm-svn: 368066