[CodeGen] Use the zero initializer instead of storing an all zero representation.

Summary:
This change avoids the overhead of storing, and later crawling,
an initializer list of all zeros for arrays. When LLVM
visits this (llvm/IR/Constants.cpp) ConstantArray::getImpl()
it will scan the list looking for an array of all zero.

We can avoid the store, and short-cut the scan, by detecting
all zeros when clang builds-up the initialization representation.

This was brought to my attention when investigating PR36030


Reviewers: majnemer, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42549

llvm-svn: 324776