Silence a MSVC warning in class Tie

With warning level 4 MSVC complains that a default
assignment operator could not be generated due to
member 'file' is a reference (warning C4512).

Use a pointer instead of a reference and move
struct Tie outisde class Logger while there.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>