Refactor ownership in dex_ir

Remove some unnecessary generality, consolidating and simplifying the
interface to Header and moving all the construction code out of dex_ir.
This makes item ownership exclusively controlled by the collections
themselves, preparing for in-place construction.

Bug: 33017139
Test: make -j 40 test-art-host-gtest
Change-Id: Ice461ae89ef9f8bed3350780e8dd6283bc6eca1b