[Demangle] fix windows tests

[nickdesaulniers]

Summary

My reland of https://reviews.llvm.org/D148546 has caused a few windows
demangler tests to fail when run with -DLLVM_ENABLE_EXPENSIVE_CHECKS=ON
on windows.

I have a sneaking suspicion that MSVC's
std::string_view::iterator::operator* may be missing a nullptr check.

Link: https://lab.llvm.org/buildbot/#/builders/42/builds/9723/steps/7/logs/stdio

[nickdesaulniers]

llvm/lib/Demangle/MicrosoftDemangle.cpp

811

I think this is potentially backwards. I'll add some asserts and flip it.

[ayzhao]

LGTM, all the failing tests seem to be passing on my Windows workstation.

[nickdesaulniers]

• add assert, change Count

[thakis]

This caused https://github.com/llvm/llvm-project/issues/63740

llvm/lib/Demangle/MicrosoftDemangle.cpp

SymbolNode *Demangler::demangleMD5Name(std::string_view &MangledName) {
   // mangled name.
   // An MD5 mangled name is ??@ followed by 32 characters and a terminating @.
   size_t MD5Last = MangledName.find('@', strlen("??@"));
   if (MD5Last == std::string_view::npos) {
     Error = true;
     return nullptr;
   }
   const char *Start = &*MangledName.begin();
+  const size_t StartSize = MangledName.size();
   MangledName.remove_prefix(MD5Last + 1);
 
   // There are two additional special cases for MD5 names:
   // 1. For complete object locators where the object name is long enough
   //    for the object to have an MD5 name, the complete object locator is
   //    called ??@...@??_R4@ (with a trailing "??_R4@" instead of the usual
   //    leading "??_R4". This is handled here.
   // 2. For catchable types, in versions of MSVC before 2015 (<1900) or after
   //    2017.2 (>= 1914), the catchable type mangling is _CT??@...@??@...@8
   //    instead of_CT??@...@8 with just one MD5 name. Since we don't yet
   //    demangle catchable types anywhere, this isn't handled for MD5 names
   //    either.
   consumeFront(MangledName, "??_R4@");
 
-  std::string_view MD5(Start, &*MangledName.begin() - Start);
+  const size_t Count = MangledName.size() - StartSize;
+  std::string_view MD5(Start, Count);
   SymbolNode *S = Arena.alloc<SymbolNode>(NodeKind::Md5Symbol);
   S->Name = synthesizeQualifiedName(Arena, MD5);
 
   return S;
 }
 
 SymbolNode *Demangler::demangleTypeinfoName(std::string_view &MangledName) {
   assert(llvm::itanium_demangle::starts_with(MangledName, '.'));

 char *llvm::microsoftDemangle(const char *MangledName, size_t *NMangled,
                               char *Buf, size_t *N,
                               int *Status, MSDemangleFlags Flags) {
   Demangler D;
 
   std::string_view Name{MangledName};
   SymbolNode *AST = D.parse(Name);
   if (!D.Error && NMangled)
-    *NMangled = &*Name.begin() - MangledName;
+    *NMangled = Name.empty() ? 0 : &*Name.begin() - MangledName;
 
   if (Flags & MSDF_DumpBackrefs)
     D.dumpBackReferences();
 
   OutputFlags OF = OF_Default;
   if (Flags & MSDF_NoCallingConvention)
     OF = OutputFlags(OF | OF_NoCallingConvention);
   if (Flags & MSDF_NoAccessSpecifier)