Auto merge of #142903 - cjgillot:local-def-path-hash, r=compiler-errors

Only inherit local hash for paths

`DefPathHash`, as the counterpart of `DefId` that is stable across compiler invocations, is comprised of 2 parts. The first one is the `StableCrateId`, stable form of `CrateNum`. The second is 64 complementary bits to identify the crate-local definition.

The current implementation always hashes the full 128 bits when (1) trying to create a new child `DefPathHash` or (2) hashing a `CrateNum` or a `LocalDefId`. But we only need half that information: `LocalDefId` means that the `StableCrateId` is always the current crate's ; `CrateNum` means that we do not care about the local part.

As stable hashing is very hot in the query system, in particular hashing definitions, this is a big deal.

We still want the local part to change when the `StableCrateId` changes, to make incr-compilation errors less painful, ie. increase the likelihood that if will magically disappear by changing some code.

This PR sprinkles some `#[inline]` attributes on small functions that appeared in profiles.

Author: bors