gdb/dwarf: move cooked_index_storage to cooked-index-storage.{h,c}
cooked_index_storage is currently declared in `cooked-index.h` and
implemented in `read.c`. Move all that to new
`cooked-index-storage.{h,c}` files.
Change-Id: I2a07eb446d8a07b15c5664dfe01e3a820cdd45be
Approved-By: Tom Tromey <tom@tromey.com>