State that incremental compilation assumes the same compiler

[isHeSatoshi]

Fixes #1860.

The chapter describes the red-green algorithm without saying that it assumes the compiler itself does not change between the previous and the current compilation. That is an easy assumption to miss while reading a guide about how to build and change the compiler: if a query's implementation changes, the same inputs no longer produce the same result, and the saved query DAG stops meaning anything.

The issue also asks whether it works anyway when you change rustc. It does not get the chance to, because rustc discards the cache rather than trying to cope with it.

This adds a note at the top of the chapter covering both points:

• the assumption is that the same compiler is used for both compilations;
• each incremental cache file starts with a header containing the compiler's version string, which encodes the git commit hash of the compiler that wrote it, and a file whose header does not match is treated as out of date.

Where this comes from, for review:

• rustc_incremental/src/persist/file_format.rs writes the header as FILE_MAGIC, a HEADER_FORMAT_VERSION, and the result of rustc_version(sess), which is sess.cfg_version (with a RUSTC_FORCE_RUSTC_VERSION override that tests use to force a mismatch on demand). Its module docs state that "compatible compiler version" means "exactly the same compiler version", since the header encodes the git commit hash of the compiler, and that being conservative costs little because the cache can always just be ignored.
• A header mismatch surfaces as OpenFileError::NotFoundOrHeaderMismatch, which load_dep_graph maps to LoadResult::DataOutOfDate, documented as "The file either didn't exist or was produced by an incompatible compiler version".
• setup_dep_graph handles DataOutOfDate by calling invalidate_old_session_dir and starting from an empty dep graph.

No new links are added.

[rustbot]

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