debuginfo: Add script that allows to conveniently start LLDB in "rust-mode"

[michaelwoerister]

This PR adds the rust-lldb script (feel free to bikeshed about the name).
The script will start LLDB and, before doing anything else, load LLDB type summaries that will make LLDB print values with Rust syntax. Just use the script like you would normally use LLDB:

rust-lldb executable-to-debug --and-any-other-commandline --args 

The script will just add one additional commandline argument to the LLDB invocation and pass along the rest of the arguments to LLDB after that.

Given the following program...

fn main() {
    let x = Some(1u);
    let y = [0, 1, 2i];
    let z = (x, y);

    println!("{} {} {}", x, y, z);
}

...without the 'LLDB type summaries', values will be printed something like this...

(lldb) p x
(core::option::Option<uint>) $3 = {
   = (RUST$ENUM$DISR = Some)
   = (RUST$ENUM$DISR = Some, 1)
}
(lldb) p y
(long [3]) $4 = ([0] = 0, [1] = 1, [2] = 2)
(lldb) p z
((core::option::Option<uint>, [int, ..3])) $5 = {
   = {
     = (RUST$ENUM$DISR = Some)
     = (RUST$ENUM$DISR = Some, 1)
  }
   = ([0] = 0, [1] = 1, [2] = 2)
}

...with the 'LLDB type summaries', values will be printed like this:

(lldb) p x
(core::option::Option<uint>) $0 = Some(1)
(lldb) p y
(long [3]) $1 = [0, 1, 2]
(lldb) p z
((core::option::Option<uint>, [int, ..3])) $2 = (Some(1), [0, 1, 2])

The 'LLDB type summaries' used by the script have been in use for a while in the LLDB autotests but I still consider them to be of alpha-version quality. If you see anything weird when you use them, feel free to file an issue.

The script will use whatever Rust "installation" is in PATH, so whichever rustc will be called if you type rustc into the console, this is the one that the script will ask for the LLDB extension module location. The build system will take care of putting the script and LLDB python module in the right places, whether you want to use the stage1 or stage2 compiler or the one coming with make install / rustup.sh.

Since I don't have much experience with the build system, Makefiles and shell scripts, please look these changes over carefully.

[alexcrichton]

When I included this in the rollup I think that this accidentally caused stage1 to always be rebuilt whenever make was invoked. You can see the fix I applied at the end of the rollup: alexcrichton@c4fa21d

r=me with the fix!

[michaelwoerister]

I don't think that this error has something to do with my changes.
I did a rebase and added a small fix to the LLDB pretty printer.