A few super-minor compiler perf improvements

[brianrourkeboll]

Description

I saw some of these show up when I did a quick profile of FSharp.Compiler.ComponentTests, which take the longest to run of all the tests.

• Override ToString on BuildPhase; the automatic printf-based implementation does a lot more work than it needs to for a fieldless DU.

  Example

  

  https://github.com/dotnet/fsharp/blob/fec36c9fe756063c8d21331b338710f3c4a501b8/src/Compiler/Driver/ParseAndCheckInputs.fs#L410-L428

  I guess I could have just changed the above to this instead:

  -Activity.Tags.buildPhase, BuildPhase.Parse.ToString()
  +Activity.Tags.buildPhase, nameof BuildPhase.Parse
  

  But maybe it's best to speed up any potential future stringifications anyway, just in case.

• Cache the delegate passed into ConcurrentDictionary.GetOrAdd where possible. See #14582, fsharp/fslang-suggestions#1083, etc.

• Remove a redundant StringBuilder in one place.

Checklist

• [x] Release notes entry updated.

[github-actions[bot]]

:heavy_exclamation_mark: Release notes required

---

:white_check_mark: Found changes and release notes in following paths:

Change path	Release notes path	Description
src/FSharp.Core	docs/release-notes/.FSharp.Core/8.0.400.md
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/8.0.400.md