[PATCH] D128947: [Lex] Introduce `PPCallbacks::LexedFileChanged()` preprocessor callback

[Argyrios Kyrtzidis via Phabricator via cfe-commits]

akyrtzi created this revision.
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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.

For `LexedFileChanged()` take the opportunity to also provide information about the prior `FileID` the `Lexer` moved from, even when
entering a new file.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D128947

Files:
  clang/include/clang/Basic/SourceManager.h
  clang/include/clang/Lex/PPCallbacks.h
  clang/lib/Basic/SourceManager.cpp
  clang/lib/Frontend/DependencyFile.cpp
  clang/lib/Lex/PPLexerChange.cpp

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