Debug Info: use createAndAddDIE for newly-created Subprogram DIEs.

More patches will be submitted to convert "new DIE(" to use createAddAndDIE in
DwarfCompileUnit.cpp. This will simplify implementation of addDIEEntry where
we have to decide between ref4 and ref_addr, because DIEs that can be shared
across CU will be added to a CU already.

Reviewed off-list by Eric.