Clean up uses of the unstable dwarf_version option

- Consolidate calculation of the effective value.
- Check the target `DebuginfoKind` instead of using `is_like_msvc`.

compiler/rustc_session/src/options.rs

@@ -1803,6 +1803,7 @@ options! {
         "output statistics about monomorphization collection"),
     dump_mono_stats_format: DumpMonoStatsFormat = (DumpMonoStatsFormat::Markdown, parse_dump_mono_stats, [UNTRACKED],
         "the format to use for -Z dump-mono-stats (`markdown` (default) or `json`)"),
+    #[rustc_lint_opt_deny_field_access("use `Session::dwarf_version` instead of this field")]
     dwarf_version: Option<u32> = (None, parse_opt_number, [TRACKED],
         "version of DWARF debug information to emit (default: 2 or 4, depending on platform)"),
     dylib_lto: bool = (false, parse_bool, [UNTRACKED],