[dotnet-linker] Use [DynamicDependency] attributes instead of manual marking when optimizing generated code.

[rolfbjarne]

This makes it easier to move this code out of a custom linker step in the future.

Also simplify a few things:

• There's no need to compute whether the code is optimizable, because the result
  is never used.
• Unify code to determine whether the InlineIsARM64CallingConvention optimization
  is to be applied.
• Any code that modifies the current assembly now returns a boolean saying so (so
  that we know if the current assembly has to be saved).

Contributes towards #17693.

[Copilot]

Pull request overview

This PR migrates the “binding optimizer” from a MarkHandler-based approach toward an AssemblyModifierStep (pre-mark) flow, aligning with the longer-term goal of reducing reliance on custom linker steps and enabling future trimmer/NativeAOT scenarios.

Changes:

• Introduces a new OptimizeGeneratedCodeStep (assembly modifier) and wires it into the SDK targets behind a DynamicDependency-based switch.
• Refactors OptimizeGeneratedCodeHandler to use a shared OptimizeGeneratedCodeData context and to report whether IL was modified.
• Extends AssemblyModifierStep with a ProcessMethod hook + helper to iterate methods, and updates smart-enum preservation to use the helper.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tools/linker/CoreOptimizeGeneratedCode.cs	Refactors optimizer logic to be data-driven/static and adds modification tracking + shared data carrier.
tools/dotnet-linker/Steps/AssemblyModifierStep.cs	Adds method-level processing hook and helper (ProcessMethods) for modifier steps.
tools/dotnet-linker/PreserveSmartEnumConversionsStep.cs	Simplifies type processing by reusing ProcessMethods and overrides the new method hook.
tools/dotnet-linker/OptimizeGeneratedCodeStep.cs	New pre-mark step that runs the generated code optimizer as an assembly modifier.
tools/common/Target.cs	Centralizes the ABI-specific “inline ARM64 calling convention” determination and caches it.
dotnet/targets/Xamarin.Shared.Sdk.targets	Adds build properties and conditions to choose between the old MarkHandler and new pre-mark step.

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• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

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• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
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🚀 [CI Build #fd74549] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 156 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 6 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 11 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ sharpie: All 1 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

macOS tests

✅ Tests on macOS Monterey (12): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Ventura (13): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sonoma (14): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sequoia (15): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Tahoe (26): All 5 tests passed. Html Report (VSDrops) Download

Linux Build Verification

✅ Linux build succeeded

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