This PR makes `rustc` emit field names for tuple fields in DWARF. Formerly there was no way of directly accessing the fields of a tuple in GDB and LLDB since there is no C/C++ equivalent to this. Now, the debugger sees the name `__{field-index}` for tuple fields. So you can type for example `some_tuple_val.__2` to get the third tuple component.
When pretty printers are used (e.g. via `rust-gdb` or `rust-lldb`) these artificial field names will not clutter tuple rendering (which was the main motivation for not doing this in the past).
Solves #21948.
b9ed9e2a32