Rollup merge of #50174 - nnethercote:FxHashMap-Interner, r=michaelwoerister

Use FxHashMap in syntax_pos::symbol::Interner::intern.

Because it's faster than HashMap.

This change reduces the time taken for a few of the rustc-perf
benchmarks, mostly the small ones, by up to 5%.
```
coercions
        avg: -1.3%      min: -5.5%      max: -0.0%
helloworld-check
        avg: -2.3%      min: -3.5%      max: -1.8%
deeply-nested-check
        avg: -1.4%      min: -3.2%      max: -0.5%
tuple-stress-opt
        avg: -0.7%      min: -2.0%      max: -0.1%
unify-linearly-check
        avg: -1.2%      min: -1.9%      max: -0.6%
coercions-check
        avg: -0.8%      min: -1.3%      max: -0.4%
unused-warnings-check
        avg: -1.0%      min: -1.3%      max: -0.8%
deeply-nested-opt
        avg: -0.5%      min: -1.2%      max: -0.2%
deeply-nested
        avg: -0.7%      min: -1.2%      max: -0.4%
helloworld
        avg: -0.8%      min: -1.1%      max: -0.7%
tuple-stress-check
        avg: -0.5%      min: -1.0%      max: -0.1%
unused-warnings
        avg: -0.8%      min: -1.0%      max: -0.7%
unused-warnings-opt
        avg: -0.8%      min: -1.0%      max: -0.7%
coercions-opt
        avg: -0.5%      min: -1.0%      max: -0.1%
helloworld-opt
        avg: -0.7%      min: -1.0%      max: -0.6%
```

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