Stop compiling LLVM with asserts

[brson]

LLVM has a lot of asserts, and we build them. In a hypothetical Rust production build maybe we want to not do that.

It's at least worth investigating the perf impact.

[bblum]

I ran a fresh build of all stages (on 45f588e) in both ways. With assertions turned on it took 20m46s, and with them disabled it took 17m40s (== 85%).

[metajack]

I think this ended up in production ready during the initial sweep. Nominating to remove milestone. This doesn't seem related to maturity.

[pnkfelix]

agreed to demilestone

[steveklabnik]

Triage: we still build with assertions, but we have a flag to build without.

[steveklabnik]

6b95d8b

This makes the default configuration fully optimized, with ... no llvm asserts,