Optimize the Regex source generator's handling of Compilation objects.

[teo-tsirpanis]

This PR optimizes the Regex source generator's pipeline in a similar way to what was done in #64579.

The Compilation object was mostly removed from the pipeline, with its presence being restricted to an IncrementalValueProvider<bool> that contains whether unsafe code is allowed, which is seamlessly combined to the main pipeline without needing a custom comparer.

The GetRegexTypeToEmit method was merged with the GetSemanticTargetForGeneration method and moved earlier in the pipeline, according to feedback from the linked PR.

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Issue Details

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This PR optimizes the Regex source generator's pipeline in a similar way to what was done in #64579.

The Compilation object was mostly removed from the pipeline, with its presence being restricted to an IncrementalValueProvider<bool> that contains whether unsafe code is allowed, which is seamlessly combined to the main pipeline without needing a custom comparer.

The GetRegexTypeToEmit method was merged with the GetSemanticTargetForGeneration method and moved earlier in the pipeline, according to feedback from the linked PR.

Author:	teo-tsirpanis
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	-

[stephentoub]

@sharwell, @chsienki, please review.

[chsienki]

Thanks @teo-tsirpanis, this looks like a nice improvement! I've written my thoughts on the attribute lookup change.

Will leave it to @stephentoub to decide, but maybe for this PR we should leave the old behavior using GetBestTypeByMetadataName and consider being more permissive in a separate change.

[teo-tsirpanis]

PR feedback was addressed; we still use GetBestTypeByMetadataName.

[stephentoub]

Thanks.