Sync with ClangSharp and regenerate against LLVM 21.1.8

[tannergooding]

Brings LLVMSharp back in line with its sibling ClangSharp and retargets the bindings to LLVM 21.1.8 (matching the Clang version ClangSharp targets).

Infrastructure sync

Props/targets, CI workflow, scripts, .editorconfig, and README are brought in line with ClangSharp, adjusting for LLVMSharp's agnostic-only packaging (no per-RID runtime packages, no win32metadata regression test):

• Adopt the PACKAGE_PUBLISH_MODE packaging flow (Directory.Build.props/.targets), enable the AOT/single-file analyzers for .NETCoreApp, and fix the .editorconfig file header to the standard .NET Foundation text.
• Split the pkg upload into its own CI artifact, modernize the signing job (Azure login + prerelease sign tool), and drop the retired nightly-feed publish jobs / stale badge + feed URL.
• Bump Microsoft.NET.Test.Sdk (18.0.1) and NUnit3TestAdapter (6.1.0).
• Add .github/copilot-instructions.md documenting the repo layout (hand-written vs generated), build/test, and the regen workflow + gotchas.

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Regenerate bindings to LLVM 21.1.8

Bump the libLLVM pin and VersionPrefix to 21.1.8 and regenerate the raw interop.

• Refresh the anonymous-enum remaps to the new header line numbers, add the LLVMGEPNoWrapFlags remap/enum, and pull in llvm-c/LLJITUtils.h (the Orc DumpObjects APIs moved there from LLJIT.h).
• Make GenerateLLVM.rsp self-contained with --file-directory and --libraryPath libLLVM, since -- unlike ClangSharp -- this repo has no generator project / launchSettings.json to supply them (previously the P/Invokes generated with an empty [DllImport("")]).

API surface changes (all genuine upstream LLVM 21 changes):

• Added: LLVMConstDataArray, LLVMGetRawDataValues, LLVMReplaceArrays, LLVMGet/SetICmpSameSign, several DIBuilder APIs, LLVMOrcCreateNewThreadSafeContextFromLLVMContext, LLVMDISubprogramReplaceType; new enum members (LLVMAtomicRMWBinOpFMaximum/FMinimum, new metadata kinds) and the new LLVMGEPNoWrapFlags enum.
• Removed: LLVMConstMul/LLVMConstNSWMul/LLVMConstNUWMul (mul is no longer a constant expression upstream -- no replacement) and LLVMOrcThreadSafeContextGetContext, along with the three hand-written const-mul wrappers in Extensions/LLVMValueRef.cs. LLVMLandingPadClauseTy is dropped (removed from the headers).
• The generated struct-layout tests are removed since generate-disable-runtime-marshalling now suppresses them (ClangSharp is the same -- it ships zero generated interop struct tests).

Notes / callouts

• LLVMAttributeIndex / LLVMJITSymbolGenericFlags kept as int enums. Generator 21.1.8.3 now preserves the headers' U literal suffixes (0U, 1U << n), which break int enums (CS0266), but the repo's unsuppressed CA1028 requires Int32-underlying enums (so uint isn't viable either). Both enums' members/values are unchanged from 20.1.2, so they're kept in their committed int form with only the version header bumped -- documented as the one expected post-regen manual fixup.
• Target aggregators restored to the full 20-target list. The clang+llvm-21.1.8-x86_64-pc-windows-msvc release binary used for regen was configured with a reduced LLVM_TARGETS_TO_BUILD, which would have silently dropped 13 backends from the InitializeAll* aggregators (they're static inline in Target.h, expanded over Config/*.def). Restored to the canonical set; documented as a regen gotcha.
• The libLLVMSharp-helper interop (GenerateLLVMSharp.rsp -> llvmsharp/) was not regenerated -- it needs the built native helper, unavailable in this environment.

Build is clean (0 warnings, TreatWarningsAsErrors on) and all 2587 tests pass.

Note

This PR body was drafted by Copilot.