libexpr: Actually cache line information in PosTable (backport #13211)

[mergify]

Motivation

Previous code had a sneaky bug due to which no caching
actually happened:

auto linesForInput = (*lines)[origin->offset];

That should have been:

auto & linesForInput = (*lines)[origin->offset];

See 1.

Now that it also makes sense to make the cache bound in size
in order not to memoize all the sources without freeing any memory.
The default cache size has been chosen somewhat arbitrarily to be ~64k
origins. For reference, 25.05 nixpkgs has ~50k .nix files.

Simple benchmark:

let
  pkgs = import <nixpkgs> { };
in
builtins.foldl' (acc: el: acc + el.line) 0 (
  builtins.genList (x: builtins.unsafeGetAttrPos "gcc" pkgs) 10000
)

(After)

$ hyperfine "result/bin/nix eval -f ./test.nix"
Benchmark 1: result/bin/nix eval -f ./test.nix
  Time (mean ± σ):     292.7 ms ±   3.9 ms    [User: 131.0 ms, System: 120.5 ms]
  Range (min … max):   288.1 ms … 300.5 ms    10 runs

(Before)

hyperfine "nix eval -f ./test.nix"
Benchmark 1: nix eval -f ./test.nix
  Time (mean ± σ):     666.7 ms ±   6.4 ms    [User: 428.3 ms, System: 191.2 ms]
  Range (min … max):   659.7 ms … 681.3 ms    10 runs

If the origin happens to be a all-packages.nix or similar in size then the
difference is much more dramatic.

Context

#12645 (comment)

@roberth As it turns out the PosTable was indeed supposed to do the caching, but didn't due to the bug I described above.

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This is an automatic backport of pull request #13211 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4711720 has failed:

On branch mergify/bp/2.28-maintenance/pr-13211
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	both modified:   src/libutil/include/nix/util/lru-cache.hh

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[xokdvium]

@roberth, to backport this you'd also need other patches I did for the LRUCache.

[roberth]

I think the risk is covered, but not sure if worth the effort.