[flang][nfc] Move `Semantics` from `FrontendAction` to `CompilerInstance`

`CompilerInstance` is a more appropriate place for a key component of
the frontend like `Semantics`.

This change opens a path for us to introduce new frontend actions that
will also run semantics, but for which inheriting from
`PrescanAndSemaAction` wouldn't make much sense. For example, for
code-gen actions we plan to introduce a dedicate hierarchy of action
classes.

I've also added a doxyment for `CompilerInstance` to add a bit of
context for this change (and also make future refactoring more informed).
As `CompilerInstance` in Flang has been inspired by its counterpart in
Clang, this comment is roughly a verbatim copy of the comment in Clang
(with some adjustments from me). Credits to Daniel Dunbar for the great
design and the original comment.

Differential Revision: https://reviews.llvm.org/D108035