Variable auto-init: also auto-init alloca

Variable auto-init: also auto-init alloca

Summary:
alloca isn’t auto-init’d right now because it’s a different path in clang that
all the other stuff we support (it’s a builtin, not an expression).
Interestingly, alloca doesn’t have a type (as opposed to even VLA) so we can
really only initialize it with memset.

<rdar://problem/49794007>

Subscribers: jkorous, dexonsmith, cfe-commits, rjmccall, glider, kees, kcc, pcc

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60548

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@358243 91177308-0d34-0410-b5e6-96231b3b80d8