[Demangle] fix windows tests

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

Reviewed By: ayzhao

Differential Revision: https://reviews.llvm.org/D148852