librustc: Implement #[disable_unwinding] and use it in librustc to avoid falling off FastISel so much

[pcwalton]

This is a 1.9x speedup in LLVM passes for unoptimized builds.

r? @graydon

[graydon]

Hm. Maybe! A few questions:

• Is it intentional that this rolls in the work in librustc: Make unqualified identifier searches terminate at the nearest ... #4345 ? Last time I tested that it broke the testsuite. In any case, seems plausibly unintentional in this pull req.
• I don't quite get the difference between begin_unwind and begin_unwind_ ,and why both call raise here.
• Won't this iloop if there's no handler, as a missing handler calls fail, which translates as a call to begin_unwind, no?
• It should probably be called disable_unwind_dtors or something, no? I mean, it is still very much unwinding, in terms of control-flow; just not running dtors while it unwinds (until it gets to the catch in the runtime).

[pcwalton]

• No, that's not intentional, although I would like to get librustc: Make unqualified identifier searches terminate at the nearest ... #4345 in before this to avoid further bitrot. I will try to make sure that librustc: Make unqualified identifier searches terminate at the nearest ... #4345 doesn't break the test suite before merging.
• I believe begin_unwind is legacy. Will comment out to be sure.
• I believe I fixed that. begin_unwind does not call the handler if it's missing.
• Sure, I can change the name.

[graydon]

ok. r+ otherwise.

[catamorphism]

@pcwalton - This needs a rebase as well.

[catamorphism]

Closing old PRs.