Reinstate the code for emitting an initial debug type for a struct,

to handle the case where the struct is only forward-declared. In
this case, a temporary MDNode is not needed and not desired.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@111842 91177308-0d34-0410-b5e6-96231b3b80d8