[PATCH] Implement variable-sized alloca instrumentation.

[Kostya Serebryany]

>>! In D6055#35, @ygribov wrote:
>> Next steps would be to enable the flag by default (we'll do our part of testing too). 
>> And then it may be interesting to enable use-after-return for allocas.
> 
> And we also need to store metadata (probably just variable name) in redzone for user-friendlier reports.

Indeed so, the diagnostics could be improved. 
Let's do it before enabling the feature by default

http://reviews.llvm.org/D6055