[demangler] Fix demangling of enumerators with negative values

rdar://27527445

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 15426b2161a4680051f13dcf0824eeab602b25fa
diff --git a/src/demangle/ItaniumDemangle.h b/src/demangle/ItaniumDemangle.h
index 85694d9..afef50b 100644
--- a/src/demangle/ItaniumDemangle.h
+++ b/src/demangle/ItaniumDemangle.h
@@ -98,7 +98,7 @@
     X(BoolExpr) \
     X(StringLiteral) \
     X(LambdaExpr) \
-    X(IntegerCastExpr) \
+    X(EnumLiteral)    \
     X(IntegerLiteral) \
     X(FloatLiteral) \
     X(DoubleLiteral) \
@@ -2036,22 +2036,26 @@
   }
 };
 
-class IntegerCastExpr : public Node {
+class EnumLiteral : public Node {
   // ty(integer)
   const Node *Ty;
   StringView Integer;
 
 public:
-  IntegerCastExpr(const Node *Ty_, StringView Integer_)
-      : Node(KIntegerCastExpr), Ty(Ty_), Integer(Integer_) {}
+  EnumLiteral(const Node *Ty_, StringView Integer_)
+      : Node(KEnumLiteral), Ty(Ty_), Integer(Integer_) {}
 
   template<typename Fn> void match(Fn F) const { F(Ty, Integer); }
 
   void printLeft(OutputStream &S) const override {
-    S += "(";
+    S << "(";
     Ty->print(S);
-    S += ")";
-    S += Integer;
+    S << ")";
+
+    if (Integer[0] == 'n')
+      S << "-" << Integer.dropFront(1);
+    else
+      S << Integer;
   }
 };
 
@@ -4270,12 +4274,12 @@
     Node *T = getDerived().parseType();
     if (T == nullptr)
       return nullptr;
-    StringView N = parseNumber();
+    StringView N = parseNumber(/*AllowNegative=*/true);
     if (N.empty())
       return nullptr;
     if (!consumeIf('E'))
       return nullptr;
-    return make<IntegerCastExpr>(T, N);
+    return make<EnumLiteral>(T, N);
   }
   }
 }