debuginfo: Re-introduce the notion of line-table-only debuginfo.
debuginfo: Expose limited debuginfo in command line options
debuginfo: Add test case for limited debuginfo
auto merge of #12714 : michaelwoerister/rust/limited-debuginfo, r=alexcrichton

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