[clang-tidy] New check for safe usage of `std::optional` and like types.

This check verifies the safety of access to `std::optional` and related
types (including `absl::optional`). It is based on a corresponding Clang
Dataflow Analysis, which does most of the work. This check merely runs it and
converts its findings into diagnostics.

Differential Revision: https://reviews.llvm.org/D121120