[clang] [AArch64]: Refactor target parser to use Bitset. (PR #65423)

[via cfe-commits]

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@@ -273,7 +275,8 @@ struct ArchInfo {
   ArchProfile Profile;   // Architecuture profile
   StringRef Name;        // Human readable name, e.g. "armv8.1-a"
   StringRef ArchFeature; // Command line feature flag, e.g. +v8a
-  uint64_t DefaultExts;  // bitfield of default extensions ArchExtKind
+  Bitset<AArch64::AEK_EXTENSIONS_LAST>
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sdesmalen-arm wrote:

Instead of writing `Bitset<...>` everywhere, can you create a typedef here, which makes this easier to use in other places? e.g.
```using FeatureBitset = Bitset<...>;```
That will also make the tests and the global initializers a bit simpler.

https://github.com/llvm/llvm-project/pull/65423