Feature gate trace_macros.

Fixes #11631
auto merge of #11842 : xales/rust/feature-gates, r=cmr

Fixes #11631
auto merge of #11595 : eddyb/rust/env-et-self-no-more, r=nikomatsakis

Non-exhaustive change list:
* `self` is now present in argument lists (modulo type-checking code I don't trust myself to refactor)
* methods have the same calling convention as bare functions (including the self argument)
* the env param is gone from all bare functions (and methods), only used by closures and `proc`s
* bare functions can only be coerced to closures and `proc`s if they are statically resolved, as they now require creating a wrapper specific to that function, to avoid indirect wrappers (equivalent to `impl<..Args, Ret> Fn<..Args, Ret> for fn(..Args) -> Ret`) that might not be optimizable by LLVM and don't work for `proc`s
* refactored some `trans::closure` code, leading to the removal of `trans::glue::make_free_glue` and `ty_opaque_closure_ptr`
Upgrade LLVM

This upgrade brings commit by @eddyb to help optimizations of virtual calls in
a few places (https://github.com/llvm-mirror/llvm/commit/6d2bd95) as well as a
commit by @c-a to *greatly* improve the runtime of the optimization passes
(https://github.com/rust-lang/llvm/pull/3).

Nice work to these guys!