inttoptr alias broken by IRSymTab code

A construct like:

@_tls_array = alias i32, inttoptr (i32 44 to i32*)

(needed to link pretty much anything that was originally linked by vc++)

breaks on this change https://reviews.llvm.org/D31364

  if (!Base)
     return make_error<StringError>("Unable to determine comdat of alias!",
                                    inconvertibleErrorCode());

(Although the way I read the code, it has very little to do with comdat)