Try to unbreak Win build after 973519826edb76

Apparently some versions of the MS STL don't like constructing a
vector from a StringMapKeyIterator<>: http://45.33.8.238/win/44999/step_4.txt
It builds fine with the MS STL on my Windows box, so just sidestep the issue.

Full error for posterity:

    VC\Tools\MSVC\14.14.26428\include\xmemory(218,75):
      error: indirection requires pointer operand ('const llvm::StringMapKeyIterator<llvm::StringRef>' invalid)
          _Uses_default_construct_t<_Alloc, decltype(_Unfancy(_UDest)), decltype(*_UFirst)>())));
    VC\Tools\MSVC\14.14.26428\include\vector(1922,11):
      note: in instantiation of function template specialization 'std::_Uninitialized_copy<...>' requested here
                    return (_Uninitialized_copy(_First, _Last, _Dest, this->_Getal()));
    VC\Tools\MSVC\14.14.26428\include\vector(757,22):
      note: in instantiation of function template specialization
            'std::vector<llvm::StringRef>::_Ucopy<llvm::StringMapKeyIterator<llvm::StringRef>>' requested here
                            this->_Mylast() = _Ucopy(_First, _Last, this->_Myfirst());
    VC\Tools\MSVC\14.14.26428\include\vector(772,3):
      note: in instantiation of function template specialization
            'std::vector<llvm::StringRef>::_Range_construct_or_tidy<llvm::StringMapKeyIterator<llvm::StringRef>>' requested here
                    _Range_construct_or_tidy(_Unchecked(_First), _Unchecked(_Last), _Iter_cat_t<_Iter>{});
    ../../clang/lib/Driver/ToolChains/Arch/X86.cpp(62,30):
      note: in instantiation of function template specialization
            'std::vector<llvm::StringRef>::vector<llvm::StringMapKeyIterator<llvm::StringRef>, void>' requested here
          std::vector<StringRef> ValidArchs{ArchMap.keys().begin(),

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