[IR] Make TypeFinder aware of DIArgList values

TypeFinder did not find types under DIArgList. This resulted in a case
of invalid IR after GlobalOpt removed a global that was the only
non-DIArgList use of a struct type.

  error: use of undefined type named 'struct.S'
    call void @llvm.dbg.value(
      metadata !DIArgList([1 x %struct.S]* undef, i64 %idxprom),
      metadata !24, metadata !DIExpression([...]))

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D103306