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

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

I vaguely remember the end result of the clang patch being that internally we cloned the build target for clang but with an extra dependency on a file that registered a clang plugin via a static constructor. If we did the same thing here, we wouldn't need these patches. I may be misremembering though. @wsmoses am I misunderstanding?

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