[llvm] Add function merger to be run during LTO link with gold plugin (PR #121343)

[via llvm-commits]

eleviant wrote:

> This change looks ok to me. However, should this pass be enabled by default instead, maybe at least when optimizing for size? It's unfortunate to have to wire through options for individual passes like this. cc @aeubanks for thoughts since he has done some pass pipeline work.

My understanding is that this option should be passed by clang driver when both -flto and -fmerge-functions are enabled and otherwise should be set to false, because at the moment function merger is not being implicitly enabled by Os or any other optimization level.

https://github.com/llvm/llvm-project/pull/121343