Introduce TYPE_SPECIFIC_RUST_STUFF

Introduce TYPE_SPECIFIC_RUST_STUFF

This adds a new enum constant, TYPE_SPECIFIC_RUST_STUFF, and changes
the DWARF reader to set this on Rust types.  This will be used as a
flag in a later patch.

Note that the size of the type_specific_field bitfield had to be
increased.  I checked that this did not impact the size of main_type.