Replace old GDB and LLDB pretty-printers with new ones
which were originally written for IntelliJ Rust.
New LLDB pretty-printers support synthetic children.
New GDB/LLDB pretty-printers support all Rust types
supported by old pretty-printers, and also support:
Rc, Arc, Cell, Ref, RefCell, RefMut, HashMap, HashSet.
We were setting the same identifier for both the DW_TAG_structure_type
and the DW_TAG_variant_part. This becomes a problem when using thinlto
becauses it uses the identifier as a key for a map of types that is used
to delete duplicates based on the ODR, so one of them is deleted as a
duplicate, resulting in invalid DWARF.
The DW_TAG_variant_part isn't a standalone type, so it doesn't need
an identifier. Fix by omitting its identifier.
