[llvm] [LTO] Avoid assert fail on failed pass plugin load (PR #96691)

[Fangrui Song via llvm-commits]

MaskRay wrote:

LGTM. Ideally LTOBackend.cpp should have a better error reporting mechanism than we do not need so many `report_fatal_error`

https://github.com/llvm/llvm-project/pull/96691