[AST] AttributedType should derive type properties from the EquivalentType

[majnemer]

Summary

Using the canonical type instead of the equivalent type can result in
insufficient template instantiations.

This fixes PR31656.

include/clang/AST/Type.h

public:
   };
 
 private:
   QualType ModifiedType;
   QualType EquivalentType;
 
   friend class ASTContext; // creates these
 
-  AttributedType(QualType canon, Kind attrKind,
-                 QualType modified, QualType equivalent)
-    : Type(Attributed, canon, canon->isDependentType(),
-           canon->isInstantiationDependentType(),
-           canon->isVariablyModifiedType(),
-           canon->containsUnexpandedParameterPack()),
+  AttributedType(QualType canon, Kind attrKind, QualType modified,
+                 QualType equivalent)
+      : Type(Attributed, canon, equivalent->isDependentType(),
+             equivalent->isInstantiationDependentType(),
+             equivalent->isVariablyModifiedType(),
+             equivalent->containsUnexpandedParameterPack()),
         ModifiedType(modified), EquivalentType(equivalent) {
     AttributedTypeBits.AttrKind = attrKind;
   }
 
 public:
   Kind getAttrKind() const {
     return static_cast<Kind>(AttributedTypeBits.AttrKind);
   }
 

test/CodeGenCXX/microsoft-abi-default-cc.cpp

 // MSABI: define x86_thiscallcc void @"\01?qux@A@@QAEXXZ"
 
 void __cdecl static_baz() {}
 // GCABI-LABEL: define void @_Z10static_bazv
 // MSABI: define void @"\01?static_baz@@YAXXZ"
 void static_qux() {}
 // GCABI-LABEL: define void @_Z10static_quxv
 // MSABI: define void @"\01?static_qux@@YAXXZ"
+
+namespace PR31656 {
+template <int I>
+void __cdecl callee(int args[I]);
+// GCABI-LABEL: declare void @_ZN7PR316566calleeILi1EEEvPi(
+// MSABI: declare void @"\01??$callee@$00@PR31656@@YAXQAH@Z"(
+
+void caller() { callee<1>(0); }
+}