debuginfo: Add LLDB version detection to compiletest and fix some LLDB issues

Apart from making the build system determine the LLDB version, this PR also fixes an issue with enums in LLDB pretty printers. In order for GDB's pretty printers to know for sure if a field of some value is an enum discriminant, I had rustc mark discriminant fields with the `artificial` DWARF tag. This worked out nicely for GDB but it turns out that one can't access artificial fields from LLDB. So I changed the debuginfo representation so that enum discriminants are marked by the special field name `RUST$ENUM$DISR` instead, which works in both cases.

The PR does not activate the LLDB test suite yet.