DebugInfo: Emit only declarations (not whole definitions) of non-unit user defined...

DebugInfo: Emit only declarations (not whole definitions) of non-unit user defined types into type units

While this doesn't come up in reasonable cases currently (the only user
defined types not in type units are ones without linkage - which makes
for near-ODR violations, because it'd be a type with linkage referencing
a type without linkage - such a type can't be validly defined in more
than one TU, so arguably it shouldn't be in a type unit to begin with -
but it's a convenient way to demonstrate an issue that will become more
revalent with homed modular debug info type definitions - which also
don't need to be in type units but more legitimately so).

Precursor to the Clang change to de-type-unit (by omitting the
'identifier') types homed due to strong linkage vtables. (making that
change without this one would lead to major type duplication in type
units)

llvm-svn: 359122