[LTO] Fix incomplete optimization remarks for dead functions when...

[LTO] Fix incomplete optimization remarks for dead functions when PreOptModuleHook or PostInternalizeModuleHook is defined

In 20a895c4, we introduce `finalizeOptimizationRemarks()` to make sure we flush the diagnostic remarks file in case the linker doesn't call the global destructors before exiting.
In https://reviews.llvm.org/D73597, we add optimization remarks for removed functions for debugging or for detecting dead code.
But there is a case, if PreOptModuleHook or PostInternalizeModuleHook is defined (e.g. `--plugin-opt=emit-llvm` is passed to linker), we do not call `finalizeOptimizationRemarks()`, therefore we will get an incomplete optimization remarks file.
This patch make sure we flush the diagnostic remarks file when PreOptModuleHook or PostInternalizeModuleHook is defined.

Reviewed By: tejohnson, MaskRay

Differential Revision: https://reviews.llvm.org/D115417