[Clang] Replace include with forward declaration (NFC) (llvm#151292)

After llvm#142541.

Author: nikic

clang/include/clang/AST/Expr.h

@@ -16,7 +16,6 @@
 #include "clang/AST/APNumericStorage.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTVector.h"
-#include "clang/AST/Attr.h"
 #include "clang/AST/ComputeDependence.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclAccessPair.h"
@@ -58,6 +57,7 @@ namespace clang {
   class StringLiteral;
   class TargetInfo;
   class ValueDecl;
+  class WarnUnusedResultAttr;
 
 /// A simple array of base specifiers.
 typedef SmallVector<CXXBaseSpecifier*, 4> CXXCastPath;