[llvm-nm] Always use opaque pointers (PR55506)

Always enable opaque pointers in llvm-nm, because the tool doesn't
actually care, and this allows us to read both typed pointer and
opaque pointer bitcode files in one archive. Previously this
depended on the order inside the archive (it would work with an
opaque pointer bitcode file first, but fail with a typed pointer
bitcode file first).

Fixes https://github.com/llvm/llvm-project/issues/55506.

Differential Revision: https://reviews.llvm.org/D125751