r? @alexcrichton

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Thanks for the PR! This PR name and description however are unfortunately very light on details (e.g. there's no description at all). Can you expand a bit on what this PR is doing, what the change is fixing, and perhaps the motivation as well?

I'm sorry. I must have been more clear.
The problem is if we do not detect if lldb exists, then when calling rust-lldb,
we could have something like this:

% rust-lldb
/home/lzutao/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rust-lldb: 34: exec: lldb: not found

With this PR, the script will check if lldb exists, and error out if lldb not found.
Also, this PR favors $() subcommand syntax over `command`. Which I think is more
clear to see.

Ok thanks for the explanation, looks good to me!

@bors: r+

📌 Commit d649ff3 has been approved by alexcrichton

@bors rollup

@lzutao: 🔑 Insufficient privileges: not in try users

⌛ Testing commit d649ff3 with merge 21d5b8b...

☀️ Test successful - checks-azure
Approved by: alexcrichton
Pushing 21d5b8b to master...