Cope with an amusingly little anomaly with dependent types and

incomplete array initialization, where we have the following in a
template:

  int a[] = { 1, 2, something-value-dependent };
  // ...
  sizeof(a);

The type of "a" appears to be a non-dependent IncompleteArrayType, but
treating it as such makes the sizeof(a) fail at template definition
time. We now correctly handle this by morphing the IncompleteArrayType
into a DependentSizedArrayType with a NULL expression, indicating that
its size has no corresponding expression (and, therefore, the type is
distinct from others).

llvm-svn: 89366