[llvm] [Opt] Enable statically-linked plugin support (PR #79227)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Let me see if I understand your constraints correctly:

- You want to add a plugin to an "unmodified" opt tool.
- "Unmodified" means you can't modify any source files or build settings.
- This is currently impossible: we don't install the relevant object files, so there's no way to refer to them.  But #79205 hacks the build config to install the relevant object files as a library, so you can build your own binary.

If making the "opt" tool a library is actually helpful, I guess I'm not really against it... but please add a proper entry-point instead of abusing dlsym().

https://github.com/llvm/llvm-project/pull/79227