[NFC] Test automatic variable initialization

Summary:
r337887 started using memset for automatic variable initialization where sensible. A follow-up discussion leads me to believe that we should better test automatic variable initialization, and that there are probably follow-up patches in clang and LLVM to improve codegen. It’ll be important to measure -O0 compile time, and figure out which transforms should be in the frontend versus the backend.

This patch is just a test of the current behavior, no questions asked. Follow-up patches will tune the code generation.

<rdar://problem/42981573>

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50361

llvm-svn: 339089