Auto merge of #33476 - nikomatsakis:incr-comp-xcrate, r=mw

track incr. comp. dependencies across crates

This PR refactors the compiler's incremental compilation hashing so that it can track dependencies across crates. The main bits are:

- computing a hash representing the metadata for an item we are emitting
  - we do this by making `MetaData(X)` be the current task while computing metadata for an item
  - this naturally registers reads from any tables and things that we read for that purpose
  - we can then hash all the inputs to those tables
- tracking when we access metadata
  - we do this by registering a read of `MetaData(X)` for each foreign item `X` whose metadata we read
- hashing metadata from foreign items
  - we do this by loading up metadata from a file in the incr. comp. directory
  - if there is no file, we use the SVH for the entire crate

There is one very simple test only at this point. The next PR will be focused on expanding out the tests.

Note that this is based on top of https://github.com/rust-lang/rust/pull/33228

r? @michaelwoerister

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