Auto merge of #35854 - nikomatsakis:incr-comp-cache-hash-35549, r=mw

compute and cache HIR hashes at beginning

This avoids the compile-time overhead of computing them twice.  It also fixes
an issue where the hash computed after typeck is differen than the hash before,
because typeck mutates the def-map in place.

Fixes #35549.
Fixes #35593.

Some performance measurements suggest this `HashesMap` is very small in memory (unobservable via `-Z time-passes`) and very cheap to construct. I do see some (very minor) performance wins in the incremental case after the first run -- the first run costs more because loading the dep-graph didn't have any hashing to do in that case. Example timings from two runs  of `libsyntex-syntax` -- the (1) indicates first run, (2) indicates second run, and (*) indicates both together:

| Phase | Master | Branch |
| ---- | ---- | ---- |
| compute_hashes_map (1) | N/A | 0.343 |
| load_dep_graph (1) | 0 | 0 |
| serialize dep graph (1) | 4.190 | 3.920 |
| total (1) | 4.190 | 4.260 |
| compute_hashes_map (2) | N/A | 0.344 |
| load_dep_graph (2) | 0.592 | 0.252 |
| serialize dep graph (2) | 4.119 | 3.779 |
| total (2) | 4.71 | 4.375 |
| total (*) | 8.9 | 8.635 |

r? @michaelwoerister

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