rustc should use llvm.lifetime intrinsics to reduce stack space

[zwarich]

It recently came up on dev-servo that Rust-compiled code is using an egregious amount of stack space. One possible solution that has been brought up is that we should use llvm.lifetime.start and llvm.lifetime.end intrinsics to reduce the amount of stack space that LLVM uses for our functions: http://llvm.org/docs/LangRef.html#llvm-lifetime-start-intrinsic.

An incomplete attempt at this has been made in an earlier PR: #12004, and the numbers there were promising.

[emberian]

cc @dotdash

[dotdash]

Started working on this again.