[clang] [llvm] [AArch64] Merge duplicate extension information. (PR #92319)

[Tomas Matheson via llvm-commits]

================
@@ -56,43 +52,64 @@ class Extension<
 
     // The FMV priority
     int FMVPriority = _FMVPriority;
+
+    // Indicates if the extension is available on the command line.
+    string IsFMVOnly = _IsFMVOnly;
----------------
tmatheson-arm wrote:

```suggestion
    bit IsFMVOnly = 0;
```

https://github.com/llvm/llvm-project/pull/92319