CodeGen: Explicitly initialize structure padding in the -ftrivial-auto-var-init mode

When generating initializers for local structures in the
-ftrivial-auto-var-init mode, explicitly wipe the padding bytes with
either 0x00 or 0xAA.

This will allow us to automatically handle the padding when splitting
the initialization stores (see https://reviews.llvm.org/D57898).

Reviewed at https://reviews.llvm.org/D58188

llvm-svn: 354861