[llvm] Add support for hashing std::optional

The credit for the hashing code, taken from D138934, goes to Ramkumar
Ramachandra.  The test comes from OptionalTest.cpp and updated for
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

Differential Revision: https://reviews.llvm.org/D139240