[mlir][llvm] Align linkage enum order with LLVM (NFC) (#118484)

This change doesn't introduce any functional differences but aligns the
implementation more closely with LLVM's representation. Previously, the
code generated a lookup table to map MLIR enums to LLVM enums due to the
lack of one-to-one correspondence. With this refactoring, the generated
code now casts directly from one enum to another.

Author: xlauko

mlir/include/mlir-c/Dialect/LLVM.h

@@ -175,17 +175,17 @@ MLIR_CAPI_EXPORTED MlirAttribute mlirLLVMComdatAttrGet(MlirContext ctx,
                                                        MlirLLVMComdat comdat);
 
 enum MlirLLVMLinkage {
-  MlirLLVMLinkagePrivate = 0,
-  MlirLLVMLinkageInternal = 1,
-  MlirLLVMLinkageAvailableExternally = 2,
-  MlirLLVMLinkageLinkonce = 3,
+  MlirLLVMLinkageExternal = 0,
+  MlirLLVMLinkageAvailableExternally = 1,
+  MlirLLVMLinkageLinkonce = 2,
+  MlirLLVMLinkageLinkonceODR = 3,
   MlirLLVMLinkageWeak = 4,
-  MlirLLVMLinkageCommon = 5,
+  MlirLLVMLinkageWeakODR = 5,
   MlirLLVMLinkageAppending = 6,
-  MlirLLVMLinkageExternWeak = 7,
-  MlirLLVMLinkageLinkonceODR = 8,
-  MlirLLVMLinkageWeakODR = 9,
-  MlirLLVMLinkageExternal = 10,
+  MlirLLVMLinkageInternal = 7,
+  MlirLLVMLinkagePrivate = 8,
+  MlirLLVMLinkageExternWeak = 9,
+  MlirLLVMLinkageCommon = 10,
 };
 typedef enum MlirLLVMLinkage MlirLLVMLinkage;