[clang] [llvm] [AArch64][Driver] Fix behavior of +nofeat modifier on -mcpu (PR #203458)

[Tomas Matheson via cfe-commits]

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@@ -340,14 +340,22 @@ void AArch64::ExtensionSet::disable(ArchExtKind E) {
       disable(Dep.Later);
 }
 
+static void setEnableIfMandatory(AArch64::ExtensionSet &Exts,
+                                 AArch64::ExtensionInfo E) {
+  if (Exts.BaseArch->DefaultExts.test(E.ID))
+    Exts.Enabled.set(E.ID);
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tommat01 wrote:

I think this is too subtly different from calling `enable()`; it needs a comment saying that it is intentionally enabling the feature without marking it as touched, and why it is doing that.

https://github.com/llvm/llvm-project/pull/203458