debuginfo: Add option for cheap, line-table-only debuginfo (fixes #12280)

[michaelwoerister]

This PR brings back limited debuginfo which allows for nice backtraces and breakpoints, but omits any info about variables and types.

The -g and --debuginfo command line options have been extended to take an optional argument:
-g0 means no debug info.
-g1 means line-tables only.
-g2 means full debug info.

Specifying -g without argument is equivalent to -g2.

Fixes #12280

[thestinger]

Reminds me that I need to update the zsh completion file...

[alexcrichton]

Just a few small things, but this is awesome, nice work!

[michaelwoerister]

Thanks for taking a look at it. I'll take care of the issues first thing tomorrow morning.

[michaelwoerister]

I fixed the file permissions issues.

[pnkfelix]

hmm, it seems like this might be using some semi-flaky parts of our getopts library.

In particular:

% rustc /tmp/hw.rs -g
% rustc -g /tmp/hw.rs
error: no input filename given
% rustc -g=2 /tmp/hw.rs
error: debug info level needs to be between 0-2
% rustc -g 2 /tmp/hw.rs
% rustc -g2 /tmp/hw.rs
% 

Personally, I would prefer if -g took no argument ever (and just used a reasonable default, say 2), and --debuginfo either required its argument or used a different syntax (e.g. --debuginfo=2) to select it.