Add a shared fix-all provider that fixes a document through one editor

[tannergooding]

WellKnownFixAllProviders.BatchFixer computes a separate fixed document per diagnostic, each
against the original, then diffs and merges them — a document fork, a semantic model bind, and a
text diff per diagnostic. Where two of those text changes overlap the merge silently drops one, and
the user has to invoke fix-all again.

SyntaxEditorFixAllProvider records every one of a document's fixes into a single SyntaxEditor
instead. It mirrors Roslyn's own SyntaxEditorBasedCodeFixProvider
— same name, same shape, same pipeline: Distinct(), source order, one editor per document, and the
single-fix and fix-all paths sharing one code path so they cannot diverge.

---

CA1516's OrderedCodeFixProvider is the only caller moved, because it already hand-rolled this exact
loop — over a DocumentEditor, with a hardcoded descending order. It now takes the shared one and is
renamed to match Roslyn.

The order it pinned is no longer declared: Order sorts ascending by span start, as Roslyn does, and
goes innermost-first only when the spans actually nest — read off the spans rather than declared by
the fixer.

---

SyntaxEditor rather than DocumentEditor, because DocumentEditor.CreateAsync
awaits GetSemanticModelAsync unconditionally, whether or not the fix reads one — and CA1516 never
does. Measured on Roslyn 3.11 over 40 fresh 400-member documents per arm, run in both orders to rule
out an ordering artifact: 1.27 / 1.20 ms per document versus 0.029 / 0.025 — 44x / 48x. That is
the cold case, which is also the fix-all case: Project and Solution scope walk unopened documents.

---

Out of scope. The repo-wide migration off the batch fixer is a large diff with its own regression
risk, and does not need reviewing in the same sitting as the thing it migrates onto.

ContainingMember / ContainingType fix-all scopes need the Roslyn 4.4
DocumentBasedFixAllProvider(ImmutableArray<FixAllScope>) constructor, since GetSupportedFixAllScopes
is sealed at every version. This PR is scope-neutral — CA1516 advertises the same three before and
after — and leaves that as one constructor call.

No ForkingSyntaxEditorBasedCodeFixProvider<T> counterpart. It has 6 consumers in Roslyn against
~120 on the plain base, and the nesting shape it exists for is covered here two cheaper ways: the
innermost-first ordering, and expressing an enclosing fix through
ReplaceNode(SyntaxNode, Func<SyntaxNode, SyntaxGenerator, SyntaxNode>). It is purely additive if a
fixer ever needs a re-bound SemanticModel mid-batch, which is forking's one irreplaceable
capability.

Builds 0 errors / 0 warnings on Roslyn 3.11; Microsoft.CodeAnalysis.NetAnalyzers.UnitTests at
17,374 / 0, main's 17,364 plus the 10 added here.

Note

Drafted with agent assistance.

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[tannergooding]

Namely this more closely unifies on what Roslyn does internally so that it is easier to migrate all the other analyzers to the "better" pattern.

[Copilot]

Pull request overview

This PR adds a shared “single-editor” fix-all implementation for NetAnalyzers code fixes, aiming to avoid WellKnownFixAllProviders.BatchFixer’s per-diagnostic document forking/merging behavior and to ensure fixes within a document compose through one SyntaxEditor. It also migrates CA1516’s intrinsics fixer to the new base/provider and adds targeted regression tests for ordering and scale.

Changes:

• Add SyntaxEditorFixAllProvider to apply all diagnostics for a document through a single SyntaxEditor, plus ordering logic for nested spans.
• Add SyntaxEditorBasedCodeFixProvider to share the single-fix and fix-all pipelines via the same implementation.
• Migrate UseCrossPlatformIntrinsicsFixer off the removed OrderedCodeFixProvider, and add fix-all regression tests for deep nesting and higher diagnostic counts.

Show a summary per file

File	Description
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/SyntaxEditorFixAllProvider.cs	New shared fix-all provider built around a single SyntaxEditor and diagnostic ordering.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/SyntaxEditorBasedCodeFixProvider.cs	New base CodeFixProvider that routes single-fix and fix-all through the same editor-based pipeline.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/OrderedCodeFixProvider.cs	Removed the previous per-fixer ordered DocumentEditor fix-all base.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.CodeQuality.Analyzers/Maintainability/UseCrossPlatformIntrinsicsFixer.cs	Migrated to the new editor-based base class/provider and updated fix application pattern.
src/Microsoft.CodeAnalysis.NetAnalyzers/tests/Microsoft.CodeAnalysis.NetAnalyzers.UnitTests/SyntaxEditorFixAllProviderTests.cs	New unit tests validating ordering rules and semantic-model binding behavior.
src/Microsoft.CodeAnalysis.NetAnalyzers/tests/Microsoft.CodeAnalysis.NetAnalyzers.UnitTests/Microsoft.CodeQuality.Analyzers/Maintainability/CSharpUseCrossPlatformIntrinsicsTests.cs	Added fix-all regression tests for deep nesting and higher diagnostic counts.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 4