gdb/dwarf: use term "shard" instead of "index"

A bit more changes as in 8e745eac7db3 ("gdb/dwarf: rename
cooked_index::m_vector to m_shards").  I think it's clearer if the term
"index" is reserved for the whole thing, while "shard" or "index shard"
are used for the parts.

Change-Id: I457bb0016a70f3f9918f4a3c3977262a7801705b
Approved-By: Tom Tromey <tom@tromey.com>
2 files changed