We're shipping a rust-enabled lldb, but the "lldb" executable is not
installed into the "bin" directory by rustup.  See the discussion in
https://github.com/rust-lang-nursery/rustup.rs/pull/1492 for
background on this decision.  There, we agreed to have rust-lldb
prefer the rust-enabled lldb if it is installed.

This patch changes dist.rs to put lldb into rustlib, following what
was done for the other LLVM tools in #53955, and then fixes rust-lldb
to prefer that lldb, if it exists.

See issue #48168