Update to LLVM 7.0

[dylanmckay]

LLVM 7.0 has been branched as of 2018-08-01, with the final tag probably a couple weeks away.

The first release candidate will be tagged in a couple of days.

Here is the llvm-dev announcement, and here is a view of the commits on the branch.

Once the final release has been tagged, Rust should be updated to LLVM 7.0.

[alexcrichton]

Great!

I've started our branches:

• rust-lang/llvm
• rust-lang/lld
• rust-lang/compiler-rt
• rust-lang-nursery/compiler-builtins

I'm hoping that this'll be pretty smooth since we upgraded most of the way through the cycle! I'm testing locally now and will send a PR once green.