[clang] [RISCV][FMV] Support target_clones (PR #85786)

[Craig Topper via cfe-commits]

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@@ -378,4 +378,8 @@ def warn_missing_symbol_graph_dir : Warning<
 def err_ast_action_on_llvm_ir : Error<
   "cannot apply AST actions to LLVM IR file '%0'">,
   DefaultFatal;
+
+def err_os_unsupport_riscv_fmv : Error<
+  "fmv is currently only supported on Linux">;
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topperc wrote:

"fmv" -> "function multiversioning" in the error message to be more user friendly

https://github.com/llvm/llvm-project/pull/85786