Use an unsigned type because there seems to be no reason for it to be signed.
Speculatively qualify some llvm::make_unique calls trying to please MSVC

It was failing with this kind of error:

C:\b\build\slave\CrWinClang\build\src\third_party\llvm\lib\TableGen\TGParser.cpp(1243) : error C2668: 'llvm::make_unique' : ambiguous call to overloaded function

        C:\b\build\slave\CrWinClang\build\src\third_party\llvm\include\llvm/ADT/STLExtras.h(408): could be 'std::unique_ptr<llvm::Record,std::default_delete<_Ty>> llvm::make_unique<llvm::Record,std::string,llvm::SMLoc&,llvm::RecordKeeper&,bool>(std::string &&,llvm::SMLoc &,llvm::RecordKeeper &,bool &&)'
        with
        [
            _Ty=llvm::Record
        ]
        C:\b\depot_tools\win_toolchain\vs2013_files\win8sdk\bin\..\..\VC\include\memory(1637): or       'std::unique_ptr<llvm::Record,std::default_delete<_Ty>> std::make_unique<llvm::Record,std::string,llvm::SMLoc&,llvm::RecordKeeper&,bool>(std::string &&,llvm::SMLoc &,llvm::RecordKeeper &,bool &&)' [found using argument-dependent lookup]
        with
        [
            _Ty=llvm::Record
        ]
        while trying to match the argument list '(std::string, llvm::SMLoc, llvm::RecordKeeper, bool)'