[clang] [llvm] [llvm] Add support for building on illumos (PR #74930)

[Fangrui Song via cfe-commits]

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@@ -241,6 +241,11 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
       set(LLVM_LINKER_IS_GNULD YES CACHE INTERNAL "")
       message(STATUS "Linker detection: GNU ld")
+    elseif("${stderr}" MATCHES "(illumos)" OR
+           "${stdout}" MATCHES "(illumos)")
+      set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
+      set(LLVM_LINKER_IS_SOLARISLD_ILLUMOS YES CACHE INTERNAL "")
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MaskRay wrote:

If illumos uses an old Solaris ld, it seems that the similarity outnumbers the discrepancy and we should set `LLVM_LINKER_IS_SOLARISLD` to yes as well.

https://github.com/llvm/llvm-project/pull/74930