[clang] Recommit "Implement [[msvc::no_unique_address]] (#65675)" (PR #67199)

[Richard Smith via cfe-commits]

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@@ -1798,11 +1798,24 @@ def ArmMveStrictPolymorphism : TypeAttr, TargetSpecificAttr<TargetARM> {
   let Documentation = [ArmMveStrictPolymorphismDocs];
 }
 
-def NoUniqueAddress : InheritableAttr, TargetSpecificAttr<TargetItaniumCXXABI> {
-  let Spellings = [CXX11<"", "no_unique_address", 201803>];
+def NoUniqueAddress : InheritableAttr {
+  let Subjects = SubjectList<[NonBitField], ErrorDiag>;
+  // No spellings because instances of this attribute are created by
+  // MSNoUniqueAddress and ItaniumNoUniqueAddress
+  let Spellings = [];
+  let Documentation = [InternalOnly];
+}
+
+def MSNoUniqueAddress : InheritableAttr, TargetSpecificAttr<TargetMicrosoftCXXABI> {
   let Subjects = SubjectList<[NonBitField], ErrorDiag>;
+  let Spellings = [CXX11<"msvc", "no_unique_address", 201803>];
+  let Documentation = [MSNoUniqueAddressDocs];
+}
+
+def ItaniumNoUniqueAddress : InheritableAttr, TargetSpecificAttr<TargetItaniumCXXABI> {
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zygoloid wrote:

I don't think this attribute name makes sense: this is the standard attribute (which just happens to have no effect on the MS ABI), not something specific to the Itanium ABI.

https://github.com/llvm/llvm-project/pull/67199