[analyzer][StdLibraryFunctionsChecker] Use Optionals throughout the summary API

By using optionals, we no longer have to check the validity of types that we
get from a lookup. This way, the definition of the summaries have a declarative
form, there are no superflous conditions in the source code.

Differential Revision: https://reviews.llvm.org/D86531

Author: Gabor Marton