Implement support for DWARF version 5.

DWARF version 5 brings a number of improvements over version 4. Quoting from
the announcement [1]:

> Version 5 incorporates improvements in many areas: better data compression,
> separation of debugging data from executable files, improved description of
> macros and source files, faster searching for symbols, improved debugging
> optimized code, as well as numerous improvements in functionality and
> performance.

On platforms where DWARF version 5 is supported (Linux, primarily), this commit
adds support for it behind a new `-Z dwarf-version=5` flag.

[1]: https://dwarfstd.org/Public_Review.php

compiler/rustc_interface/src/tests.rs

@@ -733,6 +733,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(dep_info_omit_d_target, true);
     tracked!(drop_tracking, true);
     tracked!(dual_proc_macros, true);
+    tracked!(dwarf_version, Some(5));
     tracked!(fewer_names, Some(true));
     tracked!(force_unstable_if_unmarked, true);
     tracked!(fuel, Some(("abc".to_string(), 99)));