Compiler + Tooling: Span visitor improvements (#11966)

This represents several changes to improve efficiency and reduce
allocations in the `ClassifiedSpanVisitor`, `TagHelperSpanVisitor`, and
`FormattingVisitor`.

- Pool `ClassifiedSpanVisitor` instances to reduce allocations.
- Move legacy code to the `Legacy` namespace, isolating it from current
implementations.
- Avoid creating delegates in `ClassifiedSpanVisitor` and
`FormattingVisitor`.
- Add `SpanComputer` helper to avoid creating syntax nodes just to visit
them in `ClassifiedSpanVisitor` and `FormattingVisitor`.
- Use pooled array builder for formatting spans to reduce memory
pressure.
- Add streamlined (i.e. non-Legacy) versions of `GetTagHelpers` and
`GetClassifiedSpans` in tooling. These only produce the data needed to
support tooling's GetLanguageKind.
- Other small tweaks

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CI Build:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2734526&view=results
Test Insertion:
https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/645479
Toolset Run:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2734527&view=results

Author: DustinCampbell