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 ⚙ D31364 LTO: Reduce memory consumption by creating an in-memory symbol table for InputFiles. NFCI.

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

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