swift234

[Build system] Add presets for Swift inside the LLDB tree.

This is useful when building LLDB configurations from Xcode that require the
Swift sources to be inside the LLDB tree.
While we still recommend checking out LLDB as a peer of llvm, clang, and swift,
and using build-script to automate the overall build, this allows LLDB to be
built from Xcode in the same way that it is in llvm.org.