Add -Z print-sysroot commandline option to rustc.
debuginfo: Add script that allows to conveniently start LLDB in "rust-mode"
debuginfo: Fix LLDB pretty printer for enum variants with zero fields.
auto merge of #19144 : michaelwoerister/rust/lldb-scripts, r=alexcrichton

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](http://lldb.llvm.org/varformats.html) 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...
```rust
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.