Rollup merge of #26452 - michaelsproul:the-second-coming, r=pnkfelix

As per #26009 this PR implements a new collation system for extended-error metadata. I've tried to keep it as simple as possible for now, so there's no uniqueness checking and minimal modularity.

Although using a lint was discussed in #26009 I decided against this because it would require converting the AST output from the plugin back into an internal data-structure. Emitting the metadata from within the plugin prevents this double-handling. We also didn't identify this as the source of the failures last time, although something untoward was definitely happening... With that in mind I would like as much feedback as possible on this before it's merged, I don't want to break the bots again!

I've successfully built for my host architecture and I'm building an ARM cross-compiler now to test my assumptions about the various `CFG` variables. Despite the confusing name of `CFG_COMPILER_HOST_TRIPLE` it is actually the compile time target triple, as explained in `mk/target.mk`.

```
# This is the compile-time target-triple for the compiler. For the compiler at
# runtime, this should be considered the host-triple. More explanation for why
# this exists can be found on issue #2400
export CFG_COMPILER_HOST_TRIPLE
```

CC @pnkfelix @brson @nrc @alexcrichton 

Closes #25705, closes #26009.

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