Vague linkage

[Martin von Loewis]

The vague linkage proposal suggests to emit the default constructor
together with virtual table. I believe this does not work.

The standard says that a default constructor is implicitly defined
only if it is used, and that definition of that constructor may cause
an error in the program (e.g. because one of the member constructors
is not public). So when we emit the virtual table group, we'd also
need to define the default constructor, which may give an error - even
though the program is correct.

Regards,
Martin