Defer the decision whether to use the CU or TU index until after reading the unit header.

In DWARF v4 compile units go in .debug_info and type units go in
.debug_types. However, in v5 both kinds of units are in .debug_info.
Therefore we can't decide whether to use the CU or TU index just by
looking at which section we're reading from. We have to wait until we
have read the unit type from the header.

Differential Revision: https://reviews.llvm.org/D96194