[flang][driver][nfc] Move the definition of SemanticsContext (#73669)

Moves the defintion of `SemanticsContext` within the Flang driver.

Rather than in `CompilerInvocation`, semantic context fits better within
`CompilerInstance` that encapsulates the objects that are required to
run the
frontend. `CompilerInvocation` is better suited for objects
encapsulating compiler configuration (e.g. set-up resulting from user
input or host set-up).