Move producer checks to dwarf2_cu

This changes the various producer-checking functions to be methods on
dwarf2_cu.  It adds a few new caching members as well -- every one
that could reasonably be done this way has been converted, with the
only exception being a gdbarch hook.

Note the new asserts in the accessors.  Without the earlier
prepare_one_comp_unit change, these could trigger in some modes.