RFC: implement DW_OP_bit_piece

[Stan Shebs]

Tom Tromey wrote:
> I plan to check this in, but first I would appreciate comments on the
> implementation.
>
> This patch implements DW_OP_bit_piece.  This is needed now that Jakub
> has pushed in gcc patches to generate it when SRA optimization is
> performed.
>   

Your code looks reasonable, but you're missing two, uh, pieces :-)  - 
the dwarf2_tracepoint_var_ref code to compile to bytecodes, and 
human-readable description in locexpr_describe_location_1.

(Yes it's onerous, but we did want to revive tracepoints, right?)

Stan