[dllexport] Instantiate default ctor default args for explicit specializations (PR45811)

For dllexported default constructors with default arguments, we export
default constructor closures which pass in the default args. (See D8331
for a good explanation.)

For templates, that means those default args must be instantiated even
if the function isn't called. That is done by the
InstantiateDefaultCtorDefaultArgs() function, but it wasn't done for
explicit specializations, causing asserts (see bug).

Differential revision: https://reviews.llvm.org/D91089