Fix a wrong type bug in ParsedAttr::TypeTagForDatatypeData

[riccibruno]

Summary

This patch fixes a wrong type bug inside ParsedAttr::TypeTagForDatatypeData.
The details to the best of my knowledge are as follow. The incredible thing
is that everything works out just fine by chance due to a sequence of lucky
coincidences in the layout of various types.

The struct ParsedAttr::TypeTagForDatatypeData contains among other things
a ParsedType *MatchingCType, where ParsedType is just OpaquePtr<QualType>.

However the member MatchingCType is initialized in the constructor for
type_tag_for_datatype attribute as follows:

new (&ExtraData.MatchingCType) ParsedType(matchingCType);

This results in the ParsedType being constructed in the location of the
ParsedType * Later ParsedAttr::getMatchingCType do `return
*getTypeTagForDatatypeDataSlot().MatchingCType;` which instead of
dereferencing the ParsedType * will dereference the QualType inside
the ParsedType. Now this QualType in this case contains no qualifiers
and therefore is a valid Type *. Therefore getMatchingCType returns a
Type or at least the stuff that is in the first sizeof(void*) bytes of it,
But it turns out that Type inherits from ExtQualsCommonBase and that the
first member of ExtQualsCommonBase is a const Type *const BaseType. This
Type * in this case points to the original Type pointed to by the
QualType and so everything works fine even though all the types were wrong.

This bug was only found because I changed the layout of Type,
which obviously broke all of this long chain of improbable events.

include/clang/Sema/ParsedAttr.h

AvailabilityData(const AvailabilityChange &Introduced,
     : StrictLoc(Strict), Replacement(ReplaceExpr) {
     Changes[IntroducedSlot] = Introduced;
     Changes[DeprecatedSlot] = Deprecated;
     Changes[ObsoletedSlot] = Obsoleted;
   }
 };
 
 struct TypeTagForDatatypeData {
-  ParsedType *MatchingCType;
+  ParsedType MatchingCType;
   unsigned LayoutCompatible : 1;
   unsigned MustBeNull : 1;
 };
 struct PropertyData {
   IdentifierInfo *GetterId, *SetterId;
 
   PropertyData(IdentifierInfo *getterId, IdentifierInfo *setterId)
       : GetterId(getterId), SetterId(setterId) {}
public:
   const Expr *getReplacementExpr() const {
     assert(getKind() == AT_Availability && "Not an availability attribute");
     return getAvailabilityData()->Replacement;
   }
 
   const ParsedType &getMatchingCType() const {
     assert(getKind() == AT_TypeTagForDatatype &&
            "Not a type_tag_for_datatype attribute");
-    return *getTypeTagForDatatypeDataSlot().MatchingCType;
+    return getTypeTagForDatatypeDataSlot().MatchingCType;
   }
 
   bool getLayoutCompatible() const {
     assert(getKind() == AT_TypeTagForDatatype &&
            "Not a type_tag_for_datatype attribute");
     return getTypeTagForDatatypeDataSlot().LayoutCompatible;
   }