[analyzer][MallocChecker][NFC] Split checkPostCall up, deploy CallDescriptionMap

Since its important to know whether a function frees memory (even if its a
reallocating function!), I used two CallDescriptionMaps to merge all
CallDescriptions into it. MemFunctionInfoTy no longer makes sense, it may never
have, but for now, it would be more of a distraction then anything else.

Differential Revision: https://reviews.llvm.org/D68165