[globalisel][tablegen] Try again to fix builds on old MSVC's after r300964

This should fix llvm-clang-x86_64-expensive-checks-win

I reproduced the error using the following code:
namespace llvm {
// Moving this out of the llvm namespace fixes the error.
template<unsigned NumBits> class PredicateBitsetImpl {};
}
namespace {
const unsigned MAX_SUBTARGET_PREDICATES = 11;
// This works on Clang but is broken on MSVC
//     using PredicateBitset = PredicateBitsetImpl<MAX_SUBTARGET_PREDICATES>;
// Some versions emit a syntax error here ("error C2061: syntax error: identifier
// 'PredicateBitsetImpl'") but others accept it and only emit the C3646 below.
//
// This works on Clang and MSVC
using PredicateBitset = llvm::PredicateBitsetImpl<MAX_SUBTARGET_PREDICATES>;

class Foo {
private: 
    PredicateBitset A; // error C3646: 'A': unknown override specifier
};
}

llvm-svn: 300970