rustc::driver: Remove two @s
syntax::diagnostic: Remove unnecessary traits

This removes trait `handler` and `span_handler`,
and renames `HandlerT` to `Handler`, `CodemapT` to `SpanHandler`.
syntax::diagnostic: Remove @ from Emitter
syntax::codemap: Add static DUMMY_SP

It replaces `dummy_sp()`.
auto merge of #11255 : klutzy/rust/small-cleanup, r=pcwalton

This patchset removes some `@`s and unnecessary traits,
and replaces a function (`dummy_sp()`) returning constant value
by static variable.