[flang] Clean up AssignmentContext

Eliminate `at_` and use location from `SemanticsContext` instead.

Add and use Analyze functions for `std::optional` and `std::list`.

Original-commit: flang-compiler/f18@e171029ccdb9efe874cad3a3d91adcfa200a6550
Reviewed-on: https://github.com/flang-compiler/f18/pull/928
Tree-same-pre-rewrite: false