Crash less enthusiasticially on _Atomic or __restrict__ on invalid types.

Many places in Sema cannot handle isNull() types.  This is fine, because in
most places the type building code recovers by falling back to IntTy.  In
GetFullTypeForDeclarator(), this is done at the end of the getNumTypeObjects()
loop body.  This function calls BuildQualifiedType() before this fallback is
done though, so it explicitly needs to check for isNull() types.

llvm-svn: 225124