[llvm] Support forward-referenced globals with dso_local_equivalent

See https://github.com/llvm/llvm-project/issues/57815.

dso_local_equivalent would fail with an assertion on forward-referenced
globals. This is an issue that only comes up in textual IR, which is why
we've never seen this assertion with clang.

Differential Revision: https://reviews.llvm.org/D134234