[Demangle] fix deref of std::string_view::end()

[nickdesaulniers]

Summary

In D148546, I replaced much of the use of llvm::StringView w/
std::string_view. There's one important semantic difference between the
two:

In most STL containers, end() returns an iterator that refers to one
past the end of the container. But llvm::StringView::end() refers to the
last element.

Expressions such as &*my_std_string_view.end() produce the failed
assertion:

include/c++/v1/__iterator/bounded_iter.h:93: assertion
__in_bounds(__current_) failed: __bounded_iter::operator*: Attempt to
dereference an out-of-range iterator

This was caught when copying the recent downstream changes back upstream
in D148566, and is reproducible via:

$ libcxx/utils/ci/run-buildbot generic-debug-mode

when compiled with clang and clang++. The correct way to get the same
value as before without dereferencing invalid iterators is to prefer
&*my_std_string_view.rbegin() + 1.

Fix this downstream so that I might copy it back upstream in D148566.

The other instance of &*my_std_string_view.end() that I introduced in
D148546 has been fixed already in D149061.

[ashay-github]

LGTM!

[nickdesaulniers]

• git clang-format HEAD~

llvm/include/llvm/Demangle/ItaniumDemangle.h

if (Qual.empty())
       return nullptr;
 
     // extension            ::= U <objc-name> <objc-type>  # objc-type<identifier>
     if (llvm::itanium_demangle::starts_with(Qual, "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());
+        ScopedOverride<const char *> SaveFirst(First,
+                                               &*ProtoSourceName.begin()),
+            SaveLast(Last, &*ProtoSourceName.rbegin() + 1);
         Proto = parseBareSourceName();
       }
       if (Proto.empty())
         return nullptr;
       Node *Child = getDerived().parseQualifiedType();
       if (Child == nullptr)
         return nullptr;
       return make<ObjCProtoName>(Child, Proto);