[mlir][IR] Turn `FloatType` into a type interface (#118891)

This makes it possible to add new MLIR floating point types in
downstream projects. (Adding new APFloat semantics in downstream
projects is not possible yet, so parsing/printing/converting float
literals of newly added types is not supported.)

Also removes two functions where we had to hard-code all existing
floating point types (`FloatType::classof`). See discussion here:
https://discourse.llvm.org/t/rethink-on-approach-to-low-precision-fp-types/82361

No measurable compilation time changes for these lit tests:
```
Benchmark 1: mlir-opt ./mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir -split-input-file -convert-vector-to-llvm -o /dev/null
  BEFORE
  Time (mean ± σ):     248.4 ms ±   3.2 ms    [User: 237.0 ms, System: 20.1 ms]
  Range (min … max):   243.3 ms … 255.9 ms    30 runs

  AFTER
  Time (mean ± σ):     246.8 ms ±   3.2 ms    [User: 233.2 ms, System: 21.8 ms]
  Range (min … max):   240.2 ms … 252.1 ms    30 runs


Benchmark 2: mlir-opt- ./mlir/test/Dialect/Arith/canonicalize.mlir -split-input-file -canonicalize -o /dev/null
  BEFORE
  Time (mean ± σ):      37.3 ms ±   1.8 ms    [User: 31.6 ms, System: 30.4 ms]
  Range (min … max):    34.6 ms …  42.0 ms    200 runs

  AFTER
  Time (mean ± σ):      37.5 ms ±   2.0 ms    [User: 31.5 ms, System: 29.2 ms]
  Range (min … max):    34.5 ms …  43.0 ms    200 runs


Benchmark 3: mlir-opt ./mlir/test/Dialect/Tensor/canonicalize.mlir -split-input-file -canonicalize -allow-unregistered-dialect -o /dev/null
  BEFORE
  Time (mean ± σ):     152.2 ms ±   2.5 ms    [User: 140.1 ms, System: 12.2 ms]
  Range (min … max):   147.6 ms … 161.8 ms    200 runs

  AFTER
  Time (mean ± σ):     151.9 ms ±   2.7 ms    [User: 140.5 ms, System: 11.5 ms]
  Range (min … max):   147.2 ms … 159.1 ms    200 runs
```

A micro benchmark that parses + prints 32768 floats with random
floating-point type shows a slowdown from 55.1 ms -> 48.3 ms.

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