Hi,

this patch implements dynamic allocas/VLAs sanitization in ASan. 
Basically, this is implemented at compiler part in the following way:

1) For each __builtin_alloca{_with_align} increase its size and 
alignment to contain ASan redzones.
2) Poison redzones by calling __asan_alloca_poison(alloc_addr, size) 
ASan runtime library function.
3) Remember last allocated address into separate variable called 
'last_alloca_addr'. This will be used to implement unpoisoning stuff.
4) On each stackrestore/return perform dynamic stack unpoisoning by 
calling __asan_allocas_unpoison(last_alloca_addr, restored_sp) library 
function.

With this patch I was able to find two bugs in GCC itself [1], [2] as 
well as catch a bug in Radare2 [3] initially found by Clang + LibFuzzer.
I've also managed to build Chromium but didn't find any errors there.

Does this patch looks sensible for GCC? Any feedback/suggestions would 
be greatly appreciated.

Thanks,
-Maxim

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72765
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80798
[3] https://github.com/radare/radare2/issues/6918