The function SymbolFile::ParseTypes previously accepted a SymbolContext. This makes it extremely difficult to implement faithfully, because you have to account for all possible combinations of members being set in the SymbolContext. On the other hand, no clients of this function actually care about implementing this function to this strict of a standard. AFAICT, there is actually only 1 client in the entire codebase, and it is the function ParseAllDebugSymbols, which is itself only called for testing purposes when dumping information. At this call-site, the only field it sets is the CompileUnit, meaning that an implementer of a SymbolFile need not worry about any examining or handling any other fields which might be set.
By restricting this API to accept exactly a CompileUnit& and nothing more, we can simplify the life of new SymbolFile plugin implementers by making it clear exactly what the necessary and sufficient set of functionality they need to implement is, while at the same time removing some dead code that tried to handle other types of SymbolContext fields that were never going to be set anyway.