Reduce allocations in EnumsShouldHaveZeroValueAnalyzer

[Alex-Sob]

I would like to suggest PR that is reducing allocations EnumsShouldHaveZeroValueAnalyzer:

• Currently the analyzer allocates ImmutableArray to store zero value fields of an enum type. It can be removed, in the case when there are multiple zero value fields the analyzer just needs to report an error, it can use a flag for that.

• GetZeroValueFields method allocates two enumerators with Where and Cast calls, they can be removed.

• When the analyzer gets zero value field names from analyzer options it allocates array for separator new[] { '|' }, string[] and multiple string parts with string.Split, ImmutableArray and also a delegate with closure. All those allocations can be removed, particularly string.Split call can be replaced with Span-based splitting.

  In .NET 8+ there are MemoryExtensions.Split methods to split strings that are Span-based, but they are unavailable for analyzers since they target netstandard2.0. What about providing some Span-based helpers that could be used in analyzers? I'm proposing to add such a helper in [Proposal] Use Span-based methods to split strings in analyzers #50581. In this PR I'm using the same helper in EnumsShouldHaveZeroValueAnalyzer.

[Alex-Sob]

@stephentoub There are new MemoryExtensions.Split<T> methods introduced in .NET 9, but they are unavailable for analyzers since they target netstandard2.0. Do you think it's ok to provide a simplified version to be used for analyzers? For example, it could be used in EnumsShouldHaveZeroValueAnalyzer and considerably reduce allocations. By the way, it seems that Range and Index here are local versions of respective BCL types.

[Alex-Sob]

It seems like there's no test project for Analyzer.Utilities projects, so I'm not sure where to add unit tests.

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