Fix clang 5.0 by using type aliases for LLVM DenseSet/Map

When inlining the declaration for llvm::DenseSet/DenseMap in the mlir
namespace from a forward declaration, clang does not take the default
for the template parameters if their are declared later.

namespace llvm {
  template<typename Foo>
  class DenseMap;
}
namespace mlir {
  using llvm::DenseMap;
}
namespace llvm {
  template<typename Foo = int>
  class DenseMap {};
}

namespace mlir {
  DenseMap<> map;
}

PiperOrigin-RevId: 261495612