DebugInfo: Don't put types in type units if they reference internal linkage types

Doing this causes a declaration of the internal linkage (anonymous
namespace) type to be emitted in the type unit, which would then be
ambiguous as to which internal linkage definition it refers to (since
the name is only valid internally).

It's possible these internal linkage types could be resolved relative to
the unit the TU is referred to from - but that doesn't seem ideal, and
there's no reason to put the type in a type unit since it can only be
defined in one CU anyway (since otherwise it'd be an ODR violation) & so
avoiding the type unit should be a smaller DWARF encoding anyway.

This also addresses an issue with Simplified Template Names where the
template parameter could not be rebuilt from the declaration emitted
into the TU (specifically for an enum non-type template parameter, where
looking up the enumerators is necessary to rebuild the full template
name)