[asan] fix false dynamic-stack-buffer-overflow report with constantly-sized...

[asan] fix false dynamic-stack-buffer-overflow report with constantly-sized dynamic allocas, LLVM part

See the bug report at https://github.com/google/sanitizers/issues/691. When a dynamic alloca has a constant size, ASan instrumentation will treat it as a regular dynamic alloca (insert calls to poison and unpoison), but the backend will turn it into a regular stack variable. The poisoning/unpoisoning is then broken. This patch will treat such allocas as static.

Differential Revision: http://reviews.llvm.org/D21509

llvm-svn: 273888