Deduplicate polyfill type forwarding via centralized PolyfillTypeForwarders.cs

[Copilot]

Polyfill files in src/Polyfills repeated identical #else/#elif NETCOREAPP forwarding patterns with only forwarded types differing. This change consolidates forwarding declarations into one file while keeping existing conditional behavior and polyfill implementations intact.

• Centralized forwarders

  • Added src/Polyfills/PolyfillTypeForwarders.cs as the single location for assembly-level TypeForwardedTo declarations.
  • Preserves framework guards:
    • #if NETCOREAPP for runtime-provided types
    • #if NET10_0_OR_GREATER for CompilerLoweringPreserveAttribute

• Removed duplicated per-file forwarding blocks

  • Deleted forwarding-only #else / #elif NETCOREAPP branches from affected polyfill source files.
  • Left non-forwarding code paths unchanged (polyfill definitions remain where they are).

• Consistency cleanup

  • Uses fully-qualified System.Runtime.CompilerServices.TypeForwardedTo(...) uniformly (eliminates mixed short/fully-qualified forms such as IsExternalInit’s previous short form).

// src/Polyfills/PolyfillTypeForwarders.cs
#if NETCOREAPP
[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.IsExternalInit))]
[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Index))]
[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Range))]
#endif

[Copilot]

Pull request overview

This PR refactors the src/Polyfills area by consolidating assembly-level TypeForwardedTo declarations into a single, centralized file (PolyfillTypeForwarders.cs). This reduces duplicated conditional forwarding blocks across many individual polyfill implementations while preserving the existing conditional behavior for when polyfills vs. BCL types are used.

Changes:

• Added src/Polyfills/PolyfillTypeForwarders.cs to centralize all TypeForwardedTo assembly attributes (guarded by the same TFMs/preprocessor symbols).
• Removed per-file #else / #elif NETCOREAPP forwarding-only blocks from affected polyfill files, leaving the polyfill implementations unchanged.
• Standardized forwarding declarations to use fully-qualified System.Runtime.CompilerServices.TypeForwardedTo(...).

Show a summary per file

File	Description
src/Polyfills/PolyfillTypeForwarders.cs	New centralized location for assembly-level type forwarders (NETCOREAPP + NET10_0_OR_GREATER guards).
src/Polyfills/UnreachableException.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/UnconditionalSuppressMessageAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/StackTraceHiddenAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/RequiredMemberAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/Range.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/PlatformAttributes.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/NullableAttribtues.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/ModuleInitializerAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/IsExternalInit.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized (also eliminates the short-form attribute usage).
src/Polyfills/InterpolatedStringHandlerAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/InterpolatedStringHandlerArgumentAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/Index.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/ExperimentalAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/DynamicallyAccessedMemberTypes.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/DynamicallyAccessedMembersAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/CompilerLoweringPreserveAttribute.cs	Removed per-file forwarding branch; forwarding now centralized under NET10_0_OR_GREATER.
src/Polyfills/CompilerFeatureRequiredAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.
src/Polyfills/CallerArgumentExpressionAttribute.cs	Removed per-file NETCOREAPP forwarding branch; forwarding now centralized.

Copilot's findings

• Files reviewed: 19/19 changed files
• Comments generated: 0

[Youssef1313]

This shouldn't be done this way.

Each polyfill knows exactly the TFM it's polyfilling for and should do the forwarding under #else.

Even all current polyfills are under #if !NETCOREAPP, that's just a coincidence. Also it's much easier to see the type forwarding in the same file that implements the polyfill, so I can easily see that no polyfill is missing type forwarding.

[Evangelink]

Rejecting