Support/jitlink coff seh

[ikappaki]

This PR promotes the WindowsEasyEHPlugin to a library level plugin and fixes the prerequisite JITLink COFF bugs needed to make C++ exceptions work end to end through LLJIT on msys2/mingw.

The fixes were developed iteratively using jank as the test driver. Each iteration: run jank's test suite, hit a crash or link error, write a minimal lit test that reproduces the failure, diagnose the root cause, implement the fix, confirm the lit test passes, then move on to the next failure.

Tests were written with AI assistance and verified with the fail before/pass after principle. Mentioning for transparency, they could benefit from an expert eye. Feedback welcome. I leave it to you whether you'd like to keep them.

The implementation code includes comments at the API level and inside functions where the reasoning isn't obvious from the code alone.

Changes:

1. Enable JITLink for COFF x86_64 (2019be527937) - Previously LLJIT explicitly excluded COFF (UseJITLink = !TT.isOSBinFormatCOFF()), falling back to RTDyld. Test: coff-imagebase-resolution.ll (basic JITLink linking).

2. Resolve __ImageBase before ADDR32NB lowering (8fcef754b66c) - __ImageBase was left at address 0, so .pdata/.xdata image-relative offsets were computed as absolute addresses overflowing 32 bits and crashing the unwinder. Test: coff-imagebase-resolution.ll.

3. Add GOT/PLT stubs for external calls (5bb976340d29) - External DLL calls use 32bit PC relative branches that can't reach targets beyond 2GB. Without stubs, calls to DLL functions (e.g. puts) crash with out of range relocations. Test: coff-plt-stubs.ll.

4. Add ADDR32NB stubs for image relative external references (f9f8a22050de) - .xdata references the personality function via a 32bit image relative offset, but externals in DLLs are too far away. An executable stub near JIT'd code bridges the gap. Test: coff-addr32nb-stubs.ll.

5. Fix COMDAT section-definition symbol registration (59c2bca9d561) - The first symbol in a COMDAT pair wasn't registered in the graph symbol table, so .pdata relocations referencing it by index failed with "Could not find symbol at given index." Test: coff-comdat-relocation.ll.

6. Set OverrideObjectFlags for JITLink on COFF (f226c46abf37) - COFF has no hidden visibility, all externals become Exported. Without the override, hidden visibility IR symbols (e.g. __lljit_run_atexits) trigger a "Resolving symbol with incorrect flags" assertion. Test: trivial-return-zero.ll (existing test now passes).

7. Add SEHFrameRegistrationPlugin (8327fd89a8a3) — Promotes WindowsEasyEHPlugin into a library level plugin, wired into setUpGenericLLVMIRPlatform so all LLJIT users get SEH registration automatically. Test: coff-seh-registration.ll.

8. Add filtered DLLImportDefinitionGenerator (905aef3d2517) - Only forwards __imp_ prefixed symbols. Without the filter, the generator intercepts personality function lookups and creates conflicting null stubs, crashing the unwinder. Test: coff-dllimport-filter.ll.

9. Extend SEHFrameKeepAlivePass for COMDAT .pdata$* (4198c1650b54) - The pass only matched the exact name .pdata, so COMDAT sections like .pdata$comdat_fn were dead-stripped and their unwind info lost. Test: coff-comdat-pdata-keepalive.ll.

10. Remove dead findDefinedSymbolByName from GetImageBaseSymbol (1b574b035c62) - __ImageBase is always external or absolute, never defined. The scan over all defined symbols was unnecessary dead code. Test: coff-imagebase-lookup-perf.ll.

11. Add end-to-end integration tests (13c2f438225e) - C++ throw/catch through multiple JIT'd frames (with and without frame pointers), exercising the full SEH pipeline. Tests: throw-catch-mingw-seh.ll, throw-catch-mingw-seh-no-fp.ll.

Happy to answer any follow up questions regarding any of the fixes.

Thanks

[lhames]

Sorry for the delayed reply -- I'm looking at this now.