auto merge of #10676 : eddyb/rust/ast-box-in-enums, r=cmr

**Note**: I only tested on top of my #10670 PR, size reductions come from both change sets.

With this, [more enums are shrinked](https://gist.github.com/eddyb/08fef0dfc6ff54e890bc), the most significant one being `ast_node`, from 104 bytes (master) to 96 (#10670) and now to 32 bytes.

My own testcase requires **200MB** less when compiling (not including the other **200MB** gained in #10670), and rustc-stage2 is down by about **130MB**.

I believe there is more to gain by fiddling with the enums' layouts.

diff --cc src/librustc/metadata/encoder.rs
Simple merge

diff --cc src/librustc/middle/lint.rs
Simple merge

diff --cc src/librustc/middle/trans/base.rs
Simple merge

diff --cc src/librustc/middle/trans/controlflow.rs
Simple merge

diff --cc src/librustc/middle/typeck/check/mod.rs
Simple merge

diff --cc src/libsyntax/parse/mod.rs
Simple merge