[Demangle] fix another test on windows

[nickdesaulniers]

Summary

Alan spotted another test failure that was a result of
https://reviews.llvm.org/D148546 when running expensive checks tests
locally on windows.

[ayzhao]

LGTM, confirmed this fixes the remaining broken tests on Windows:

C:\src\llvm-project\build-ninja>.\bin\llvm-lit.py ..\llvm\test\Transforms\SampleProfile\remap-2.ll
llvm-lit.py: C:\src\llvm-project\llvm\utils\lit\lit\llvm\config.py:46: note: using lit tools: C:\Program Files\Git\usr\bin
-- Testing: 1 tests, 1 workers --
PASS: LLVM :: Transforms/SampleProfile/remap-2.ll (1 of 1)

Testing Time: 0.44s
  Passed: 1

C:\src\llvm-project\build-ninja>unittests\ProfileData\ProfileDataTests.exe
[==========] Running 197 tests from 10 test suites.
[----------] Global test environment set-up.

(output redacted for brevity)

[----------] Global test environment tear-down
[==========] 197 tests from 10 test suites ran. (283 ms total)
[  PASSED  ] 197 tests.

llvm/lib/ProfileData/ItaniumManglingCanonicalizer.cpp

 using llvm::itanium_demangle::Node;
 using llvm::itanium_demangle::NodeKind;
 
 namespace {
 struct FoldingSetNodeIDBuilder {
   llvm::FoldingSetNodeID &ID;
   void operator()(const Node *P) { ID.AddPointer(P); }
   void operator()(std::string_view Str) {
+    if (Str.empty())
+      ID.AddString({});
+    else
       ID.AddString(llvm::StringRef(&*Str.begin(), Str.size()));
   }
   template <typename T>
   std::enable_if_t<std::is_integral_v<T> || std::is_enum_v<T>> operator()(T V) {
     ID.AddInteger((unsigned long long)V);
   }
   void operator()(itanium_demangle::NodeArray A) {
     ID.AddInteger(A.size());
     for (const Node *N : A)