[CMake] Add first CMake cache files

Summary:
CMake cache scripts pre-populate the CMakeCache in a build directory with commonly used settings.
The CMake invocation from D61952 could look like this:

```
cmake -G Ninja -C /path/to/llvm-project/lldb/cmake/caches/Apple-lldb-osx.cmake -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi;lldb" ../llvm-project/llvm
```

Options specified on the command line will override options in the cache files (as long as caches don't use `FORCE`).
What do you think? (This is a first proposal and not set in stone.)

Reviewers: xiaobai, compnerd, JDevlieghere, aprantl, labath

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61956

llvm-svn: 361069