gdb/dwarf: rename cooked_index to cooked_index_shard

gdb/dwarf: rename cooked_index to cooked_index_shard

I propose to rename cooked_index_vector and cooked_index such that the
"main" object, that is the entry point to the index, is called
cooked_index.  The fact that the cooked index is implemented as a vector
of smaller indexes is an implementation detail.

This patch renames cooked_index to cooked_index_shard.  The following
patch renames cooked_index_vector to cooked_index.

Change-Id: Id650f97dcb23c48f8409fa0974cd093ca0b75177
Approved-By: Tom Tromey <tom@tromey.com>