Previously, LLVM was generating a ton of byte-by-byte copies,
leading to huge numbers of vregs and bloating the code. Now, using
memset, the code becomes a nice series of SSE moves instead.

This is needed for a lot of Apple libraries, as Apple tends to put a lot of
globals in dynamic libraries.

A similar approach could be used for type parameters.

Fixes #2282.

There used to be two distinct code paths.  Now there is one.

this will require more temporaries, but is probably less magical.
also, it means that borrowck matches trans better, so fewer crashes.
bonus.

Finally, stop warning about implicit copies when we are actually borrowing.

Also, one test (vec-res-add) stopped failing due to #2587, and hence I
added an xfail-test.

Fixes #3217, #2977, #3067

There is also code for ~Trait and &Trait, but these are currently (incorrectly)
synonyms for "as @Trait" and "as &Trait".

This reverts commit 0101125a.

The first case was getting treated like a variable binding, meaning that
if e had a destructor, it wouldn't run until the end of the enclosing scope.
To me it seems less confusing for let _ = e; and e; to work exactly the same
way, so now, the destructor for e runs immediately in both cases.