[MSVC Compat] Diagnose multiple default ctors for dllexport'd classes

The MS ABI emits a special default constructor closure thunk if a
default constructor has a weird calling convention or default arguments.

The MS ABI has a quirk: there can be only one such thunk because the
mangling scheme does not have room for distinct manglings.  We must
raise a diagnostic in this eventuality.

N.B.  MSVC sorta gets this right.  Multiple default constructors result
in the default constructor closure getting emitted but they seem to
get confused by which default constructors are reasonable to reference
from the closure.  We try to be a little more careful which results in
mild differences in behavior.

llvm-svn: 256661