[gn build] Add build files for LLDB

This is enough to get the lit-based tests to pass on macOS.

Doesn't yet add build targets for:
- Any LLDB unit tests
- swig bindings
- various targets not needed by lit tests

LLDB has many dependency cycles, something GN doesn't allow. For
that reason, I've omitted some dependency edges. Hopefully we can
clean up the cycles one day.

LLDB has a public/private header distinction, but mostly ignores it.
Many libraries include private headers from other modules.

Since LLDB is the first target the LLVM/GN build that uses Objective-C++
code, add some machinery to the toolchain file to handle that.

Differential Revision: https://reviews.llvm.org/D109185