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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The clang version of this only supported Bazel builds, not CMake builds, so it didn't have to worry about -DBUILD_SHARED_LIBS.  (I didn't realize this at the time because the patch didn't actually touch those bits.)

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