rustdoc is not built with debuginfo, even if `debuginfo=true`

[QuietMisdreavus]

While investigating #44613, i found that even though i set debuginfo = true in my config.toml, the symbols that came out would not include any source information for librustdoc. Backtraces for panics didn't have useful method names unless i turned off optimizations. Attempts to debug rustdoc in Visual Studio would just have a blind spot for any source information within librustdoc itself. Profiles would show mangled names for any librustdoc methods.

EDIT: I should note, a rustdoc-tool-binary-(hash).pdb was created, and Visual Studio dutifully found it and loaded it, but it still had all the problems noted above.