[Analyzer][NFC] Remove the SubEngine interface

The `SubEngine` interface is an interface with only one implementation
`EpxrEngine`. Adding other implementations are difficult and very
unlikely in the near future. Currently, if anything from `ExprEngine` is
to be exposed to other classes it is moved to `SubEngine` which
restricts the alternative implementations. The virtual methods are have
a slight perofrmance impact. Furthermore, instead of the `LLVM`-style
inheritance a native inheritance is used here, which renders `LLVM`
functions like e.g. `cast<T>()` unusable here. This patch removes this
interface and allows usage of `ExprEngine` directly.

Differential Revision: https://reviews.llvm.org/D80548