auto merge of #13301 : erickt/rust/remove-refcell-get, r=huonw

`RefCell::get` can be a bit surprising, because it actually clones the wrapped value. This removes `RefCell::get` and replaces all the users with `RefCell::borrow()` when it can, and `RefCell::borrow().clone()` when it can't. It removes `RefCell::set` for consistency. This closes #13182.

It also fixes an infinite loop in a test when debugging is on.

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

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

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

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

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

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