Rollup merge of #53973 - tromey:prefer-rust-enabled-lldb, r=alexcrichton

Rollup merge of #53973 - tromey:prefer-rust-enabled-lldb, r=alexcrichton

Have rust-lldb look for the rust-enabled lldb

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
rust-lldb to look in the sysroot and use the lldb found there, if any.

See issue #48168