Don't require -memprof-report-hinted-sizes for emitting opt remarks
during the thin link step. Invoke the handling also when opt remarks are
enabled for MemProf per OptimizationRemarkEmitter::allowExtraAnalysis.
Also, add a fallback message if we don't have the context size
information, adding tests for those new messages.
I also realized we don't currently emit these messages for MemProf with
regular LTO, and added a TODO.
2b212315f1