Having both salvageDebugInfo() and salvageDebugInfoOrMarkUndef() has led to situations when the incorrect function was called. Eliminate the potential for error by combining the two functions. First suggested in D76930.
Now all SalvageDebugInfo() calls will mark undef if the salvage attempt fails. This did not require changes to call sites, except the function name, but some tests resulted in extra debug info (undef) being generated where previously there was none.