Represent tq"" as SyntacticEmptyTypeTree rather than TypeTree()

Such representation codifies the fact that type tree that doesn't have
embedded syntactic equivalent must have been inferred or otherwise
provided by the compiler rather than specified by the end user.

Additionally it also ensures that we can still match trees without
explicit types (e.g. vals without type) after typechecking. Otherwise
the same quote couldn't be used in situations like:

    val q"val x = 42" = typecheck(q"val x = 42")