[Demangle] replace std::string_view::substr which may throw

[nickdesaulniers]

Summary

libcxxabi/src/demangle/ copies the implementation from llvm/Demangle.
libcxxabi/ cannot use potentially-throwing std::string_view::substr, so
change llvm/Demangle to avoid these function calls.

I ran into linkage failures stemming from the usage of
std::string_view::substr. substr does a bounds check and may throw.

Fixes: f198e0b594aa ("[StringView] remove dropFront")

llvm/include/llvm/Demangle/ItaniumDemangle.h

void printLeft(OutputBuffer &OB) const override {
     SubExpr->print(OB);
     OB += ".<";
     Type->print(OB);
     OB += " at offset ";
     if (Offset.empty()) {
       OB += "0";
     } else if (Offset[0] == 'n') {
       OB += "-";
-      OB += Offset.substr(1);
+      OB += std::string_view(Offset.data() + 1, Offset.size() - 1);
     } else {
       OB += Offset;
     }
     OB += ">";
   }
 };
 
 class EnclosingExpr : public Node {
public:
   template<typename Fn> void match(Fn F) const { F(Ty, Integer); }
 
   void printLeft(OutputBuffer &OB) const override {
     OB.printOpen();
     Ty->print(OB);
     OB.printClose();
 
     if (Integer[0] == 'n')
-      OB << '-' << Integer.substr(1);
+      OB << '-' << std::string_view(Integer.data() + 1, Integer.size() - 1);
     else
       OB << Integer;
   }
 };
 
 class IntegerLiteral : public Node {
   std::string_view Type;
   std::string_view Value;
 
 public:
   IntegerLiteral(std::string_view Type_, std::string_view Value_)
       : Node(KIntegerLiteral), Type(Type_), Value(Value_) {}
 
   template<typename Fn> void match(Fn F) const { F(Type, Value); }
 
   void printLeft(OutputBuffer &OB) const override {
     if (Type.size() > 3) {
       OB.printOpen();
       OB += Type;
       OB.printClose();
     }
 
     if (Value[0] == 'n')
-      OB << '-' << Value.substr(1);
+      OB << '-' << std::string_view(Value.data() + 1, Value.size() - 1);
     else
       OB += Value;
 
     if (Type.size() <= 3)
       OB += Type;
   }
 };
 

 Node *AbstractManglingParser<Derived, Alloc>::parseQualifiedType() {
   if (consumeIf('U')) {
     std::string_view Qual = parseBareSourceName();
     if (Qual.empty())
       return nullptr;
 
     // extension            ::= U <objc-name> <objc-type>  # objc-type<identifier>
     if (llvm::itanium_demangle::starts_with(Qual, "objcproto")) {
-      std::string_view ProtoSourceName = Qual.substr(std::strlen("objcproto"));
+      constexpr size_t Len = sizeof("objcproto") - 1;
+      std::string_view ProtoSourceName(Qual.data() + Len, Qual.size() - Len);
       std::string_view Proto;
       {
         ScopedOverride<const char *> SaveFirst(First, &*ProtoSourceName.begin()),
             SaveLast(Last, &*ProtoSourceName.end());
         Proto = parseBareSourceName();
       }
       if (Proto.empty())
         return nullptr;