[AST] Fix an undefine behavior when creating an empty recovery expr.

[hokein]

Summary

We forgot to initialize the NumExpr member in one of the constructors,
which leads crashes in preamble serialization.

clang/include/clang/AST/Expr.h

public:
 
   static bool classof(const Stmt *T) {
     return T->getStmtClass() == RecoveryExprClass;
   }
 
 private:
   RecoveryExpr(ASTContext &Ctx, SourceLocation BeginLoc, SourceLocation EndLoc,
                ArrayRef<Expr *> SubExprs);
-  RecoveryExpr(EmptyShell Empty) : Expr(RecoveryExprClass, Empty) {}
+  RecoveryExpr(EmptyShell Empty, unsigned NumSubExprs)
+      : Expr(RecoveryExprClass, Empty), NumExprs(NumSubExprs) {}
 
   size_t numTrailingObjects(OverloadToken<Stmt *>) const { return NumExprs; }
 
   SourceLocation BeginLoc, EndLoc;
   unsigned NumExprs;
   friend TrailingObjects;
   friend class ASTStmtReader;
   friend class ASTStmtWriter;
 };
 
 } // end namespace clang
 
 #endif // LLVM_CLANG_AST_EXPR_H

clang/lib/AST/Expr.cpp

RecoveryExpr *RecoveryExpr::Create(ASTContext &Ctx, SourceLocation BeginLoc,
   void *Mem = Ctx.Allocate(totalSizeToAlloc<Expr *>(SubExprs.size()),
                            alignof(RecoveryExpr));
   return new (Mem) RecoveryExpr(Ctx, BeginLoc, EndLoc, SubExprs);
 }
 
 RecoveryExpr *RecoveryExpr::CreateEmpty(ASTContext &Ctx, unsigned NumSubExprs) {
   void *Mem = Ctx.Allocate(totalSizeToAlloc<Expr *>(NumSubExprs),
                            alignof(RecoveryExpr));
-  return new (Mem) RecoveryExpr(EmptyShell());
+  return new (Mem) RecoveryExpr(EmptyShell(), NumSubExprs);
 }
 
 void OMPArrayShapingExpr::setDimensions(ArrayRef<Expr *> Dims) {
   assert(
       NumDims == Dims.size() &&
       "Preallocated number of dimensions is different from the provided one.");
   llvm::copy(Dims, getTrailingObjects<Expr *>());
 }

clang/test/PCH/cxx-recovery-expr.cpp

+// Test with pch.
+// RUN: %clang_cc1 -emit-pch -frecovery-ast -fallow-pch-with-compiler-errors -o %t %s
+// RUN: %clang_cc1 -include-pch %t -fno-validate-pch -emit-llvm -o - %s
+
+#ifndef HEADER
+#define HEADER
+
+int func(int);
+int s = func();
+
+#else
+
+#endif