[PATCH] [WIP] Use dynamic alloca for stack variables in ASan.

[Kuba Brecka]

>>! In D6465#16, @samsonov wrote:
> I've re-run the SPEC after the most recent changes, and still see a minor improvement in average results: this change as-is gives 4.5% improvement against the current code, and if/when we enable asan-stack-dynamic-alloca by default, the improvement against the current code is 3.5%.

Awesome!

http://reviews.llvm.org/D6465