Work around dllimport bug with exclude_from_explicit_instantiation.

When dllimport is specified on a class, and
exclude_from_explicit_instatiation is specified on a member, clang-cl
will still expect a definition to be available externally. But this is
not correct.

Surprisingly one one symbol seems to be consistently affected by this
bug. So this patch simply works around it there.

llvm-svn: 355760