[clang] [llvm] [AArch64] Add ability to list extensions enabled for a target (PR #95805)

[Tomas Matheson via cfe-commits]

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@@ -14,23 +14,36 @@
 class Extension<
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tmatheson-arm wrote:

The comment needs updating, something like:
> A SubtargetFeature that represents one or more Architecture Extensions, as defined in the Arm ARM and typically named `FEAT_*`. Each has an `AEK_*` entry in the ArmExtKind enum.

https://github.com/llvm/llvm-project/pull/95805