[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);
+}
+
 void AArch64::ExtensionSet::addCPUDefaults(const CpuInfo &CPU) {
   LLVM_DEBUG(llvm::dbgs() << "addCPUDefaults(" << CPU.Name << ")\n");
   BaseArch = &CPU.Arch;
 
   AArch64::ExtensionBitset CPUExtensions = CPU.getImpliedExtensions();
-  for (const auto &E : Extensions)
+  for (const auto &E : Extensions) {
     if (CPUExtensions.test(E.ID))
       enable(E.ID);
+    setEnableIfMandatory(*this, E);
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tommat01 wrote:

It's a bit odd to pass a reference to `*this` rather than making it a member of `ExtensionSet`

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