[gdb/symtab] Add dwarf_expr_piece.op

Add a new field "dwarf_location_atom op" to dwarf_expr_piece to keep track of
which dwarf_location_atom caused a dwarf_expr_piece to be added.

This is used in the following patch.

Tested on s390x-linux.

Approved-By: Tom Tromey <tom@tromey.com>