Qualify "Decl" type in a member named "Decl"

Author: DougGregor

clang/include/clang/AST/ASTContext.h

@@ -804,7 +804,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
 
   struct AvailabilityDomainInfo {
     FeatureAvailKind Kind = FeatureAvailKind::None;
-    Decl *Decl = nullptr;
+    clang::Decl *Decl = nullptr;
     ImplicitCastExpr *Call = nullptr;
     bool isInvalid() const { return Kind == FeatureAvailKind::None; }
   };