Upstream all of the patches for llvm-8.0svn support

[dylanmckay]

In #115, the version of Rust and LLVM is being updated.

A few patches were cherry-picked into the LLVM fork.

These should be upstreamed.

Here is a full list.

• [x] avr-rust/rust#37: DREGS_WITHOUT_Z
• [ ] avr-rust/rust#92: ret struct
• [x] avr-rust/rust#112: extra op
• [x] avr-rust/rust#117 " Can't compile when using a workspace"
• [ ] external author D53106 [SelectionDAG] Fix behavior topological ordering with regards to glued nodes.
• [x] external author D53485 [ScheduleDAGRRList] Do not preschedule the node has ADJCALLSTACKDOWN parent

[dylanmckay]

Upstreamed #37 in r346114.

Also raised LLVM bug PR39553 to track the workaround and a proper fix.

[dylanmckay]

Upstreamed #112 in r346117.

Also raised LLVM bug PR39554 to track writing of proper unit tests for FRMIDX expansion.

[dylanmckay]

Upstream #117 in r349469

[dylanmckay]

https://reviews.llvm.org/D53485 has recently landed