[clang] [llvm] [ABI][CIR][NFC] Let X86AVXABILevel carry its own count (PR #216339)

[Erich Keane via llvm-commits]

================
@@ -91,6 +91,7 @@ enum class X86AVXABILevel {
   None,
   AVX,
   AVX512,
+  NumberOfEnumEntries // must be last
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erichkeane wrote:

```suggestion
  Last = AVX512  // must be last
```

This is a pretty common pattern we have in our enums, perhaps this? 

https://github.com/llvm/llvm-project/pull/216339