[Lex] Introduce `PPCallbacks::LexedFileChanged()` preprocessor callback

This is a preprocessor callback focused on the lexed file changing, without conflating effects of line number directives and other pragmas.
A client that only cares about what files the lexer processes, like dependency generation, can use this more straightforward
callback instead of `PPCallbacks::FileChanged()`. Clients that want the pragma directive effects as well can keep using `FileChanged()`.

A use case where `PPCallbacks::LexedFileChanged()` is particularly simpler to use than `FileChanged()` is in a situation
where a client wants to keep track of lexed file changes that include changes from/to the predefines buffer, where it becomes
unnecessary complicated trying to use `FileChanged()` while filtering out the pragma directives effects callbacks.

Also take the opportunity to provide information about the prior `FileID` the `Lexer` moved from, even when entering a new file.

Differential Revision: https://reviews.llvm.org/D128947

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